Update RegistryClient to use JSON POST for createRelease endpoint

- Refactor createDraftRelease() to send JSON request body instead of multipart/form-data
- Add JsonOutput import for JSON serialization
- Build JSON request with id, version, checksum, provider fields
- Use Content-Type: application/json header
- Remove obsolete buildDraftMetadataBody() method
- Keep buildArtifactUploadBody() unchanged (Step 2 still uses multipart)

- Update tests to verify JSON format:
  - Rename test to 'should send correct JSON in two-step process'
  - Verify Content-Type: application/json header in Step 1
  - Check for JSON content instead of multipart form fields
  - Verify Step 2 still uses multipart for artifact upload

Compatible with plugin-registry API v0.13.0+ which changed
createRelease endpoint from multipart to JSON POST.

Author: pditommaso

src/main/groovy/io/nextflow/gradle/registry/RegistryClient.groovy

@@ -1,5 +1,6 @@
 package io.nextflow.gradle.registry
 
+import groovy.json.JsonOutput
 import groovy.json.JsonSlurper
 import groovy.transform.CompileStatic
 import groovy.util.logging.Slf4j
@@ -118,19 +119,33 @@ class RegistryClient {
      * @throws RegistryReleaseException if the request fails
      */
     private Long createDraftRelease(String id, String version, File file, String provider) {
+        if (!provider) {
+            throw new IllegalArgumentException("Plugin provider is required for plugin upload")
+        }
+
         def client = HttpClient.newBuilder()
             .connectTimeout(Duration.ofSeconds(30))
             .build()
 
-        def boundary = "----FormBoundary" + UUID.randomUUID().toString().replace("-", "")
-        def multipartBody = buildDraftMetadataBody(id, version, file, provider, boundary)
+        // Calculate SHA-512 checksum
+        def fileBytes = Files.readAllBytes(file.toPath())
+        def checksum = computeSha512(fileBytes)
+
+        // Build JSON request body
+        def requestBody = [
+            id: id,
+            version: version,
+            checksum: "sha512:${checksum}",
+            provider: provider
+        ]
+        def jsonBody = JsonOutput.toJson(requestBody)
 
         def requestUri = URI.create(url.toString() + "v1/plugins/release")
         def request = HttpRequest.newBuilder()
             .uri(requestUri)
             .header("Authorization", "Bearer ${authToken}")
-            .header("Content-Type", "multipart/form-data; boundary=${boundary}")
-            .POST(HttpRequest.BodyPublishers.ofByteArray(multipartBody))
+            .header("Content-Type", "application/json")
+            .POST(HttpRequest.BodyPublishers.ofString(jsonBody))
             .timeout(Duration.ofMinutes(1))
             .build()
 
@@ -206,64 +221,6 @@ class RegistryClient {
         return message.toString()
     }
 
-    /**
-     * Builds multipart body for Step 1 (draft creation with metadata only).
-     *
-     * @param id The plugin identifier
-     * @param version The plugin version
-     * @param file The plugin file (used to compute checksum)
-     * @param provider The plugin provider
-     * @param boundary The multipart boundary string
-     * @return Multipart body as byte array
-     */
-    private byte[] buildDraftMetadataBody(String id, String version, File file, String provider, String boundary) {
-        if (!provider) {
-            throw new IllegalArgumentException("Plugin provider is required for plugin upload")
-        }
-
-        def output = new ByteArrayOutputStream()
-        def writer = new PrintWriter(new OutputStreamWriter(output, "UTF-8"), true)
-        def lineEnd = "\r\n"
-
-        // Calculate SHA-512 checksum
-        def fileBytes = Files.readAllBytes(file.toPath())
-        def checksum = computeSha512(fileBytes)
-
-        // Add id field
-        writer.append("--${boundary}").append(lineEnd)
-        writer.append("Content-Disposition: form-data; name=\"id\"").append(lineEnd)
-        writer.append("Content-Type: text/plain; charset=UTF-8").append(lineEnd)
-        writer.append(lineEnd)
-        writer.append(id).append(lineEnd)
-
-        // Add version field
-        writer.append("--${boundary}").append(lineEnd)
-        writer.append("Content-Disposition: form-data; name=\"version\"").append(lineEnd)
-        writer.append("Content-Type: text/plain; charset=UTF-8").append(lineEnd)
-        writer.append(lineEnd)
-        writer.append(version).append(lineEnd)
-
-        // Add checksum field
-        writer.append("--${boundary}").append(lineEnd)
-        writer.append("Content-Disposition: form-data; name=\"checksum\"").append(lineEnd)
-        writer.append("Content-Type: text/plain; charset=UTF-8").append(lineEnd)
-        writer.append(lineEnd)
-        writer.append("sha512:${checksum}").append(lineEnd)
-
-        // Add provider field
-        writer.append("--${boundary}").append(lineEnd)
-        writer.append("Content-Disposition: form-data; name=\"provider\"").append(lineEnd)
-        writer.append("Content-Type: text/plain; charset=UTF-8").append(lineEnd)
-        writer.append(lineEnd)
-        writer.append(provider).append(lineEnd)
-
-        // End boundary
-        writer.append("--${boundary}--").append(lineEnd)
-        writer.close()
-
-        return output.toByteArray()
-    }
-
     /**
      * Builds multipart body for Step 2 (artifact upload only).
      *

src/test/groovy/io/nextflow/gradle/registry/RegistryClientTest.groovy

@@ -50,17 +50,18 @@ class RegistryClientTest extends Specification {
         def pluginFile = tempDir.resolve("test-plugin.zip").toFile()
         pluginFile.text = "fake plugin content"
 
-        // Step 1: Create draft release
+        // Step 1: Create draft release (JSON)
         wireMockServer.stubFor(post(urlEqualTo("/api/v1/plugins/release"))
             .withHeader("Authorization", equalTo("Bearer test-token"))
-            .withRequestBody(containing("id"))
-            .withRequestBody(containing("version"))
-            .withRequestBody(containing("checksum"))
+            .withHeader("Content-Type", equalTo("application/json"))
+            .withRequestBody(containing("\"id\""))
+            .withRequestBody(containing("\"version\""))
+            .withRequestBody(containing("\"checksum\""))
             .willReturn(aResponse()
                 .withStatus(200)
                 .withBody('{"releaseId": 123, "pluginRelease": {"status": "DRAFT"}}')))
 
-        // Step 2: Upload artifact
+        // Step 2: Upload artifact (multipart)
         wireMockServer.stubFor(post(urlMatching("/api/v1/plugins/release/.*/upload"))
             .withHeader("Authorization", equalTo("Bearer test-token"))
             .withRequestBody(containing("payload"))
@@ -76,7 +77,8 @@ class RegistryClientTest extends Specification {
 
         and:
         wireMockServer.verify(postRequestedFor(urlEqualTo("/api/v1/plugins/release"))
-            .withHeader("Authorization", equalTo("Bearer test-token")))
+            .withHeader("Authorization", equalTo("Bearer test-token"))
+            .withHeader("Content-Type", equalTo("application/json")))
         wireMockServer.verify(postRequestedFor(urlEqualTo("/api/v1/plugins/release/123/upload"))
             .withHeader("Authorization", equalTo("Bearer test-token")))
     }
@@ -151,38 +153,35 @@ class RegistryClientTest extends Specification {
         // Java HTTP client may convert UnknownHostException to ConnectException
     }
 
-    def "should send correct multipart form data in two-step process"() {
+    def "should send correct JSON in two-step process"() {
         given:
         def pluginFile = tempDir.resolve("test-plugin.zip").toFile()
         pluginFile.text = "fake plugin zip content"
 
-        // Step 1: Create draft with metadata
+        // Step 1: Create draft with metadata (JSON)
         wireMockServer.stubFor(post(urlEqualTo("/api/v1/plugins/release"))
             .willReturn(aResponse()
                 .withStatus(200)
                 .withBody('{"releaseId": 456}')))
 
-        // Step 2: Upload artifact
+        // Step 2: Upload artifact (multipart)
         wireMockServer.stubFor(post(urlMatching("/api/v1/plugins/release/.*/upload"))
             .willReturn(aResponse().withStatus(200)))
 
         when:
         client.release("my-plugin", "2.1.0", pluginFile, "seqera.io")
 
         then:
-        // Verify Step 1: draft creation with metadata only
+        // Verify Step 1: draft creation with JSON metadata
         wireMockServer.verify(postRequestedFor(urlEqualTo("/api/v1/plugins/release"))
             .withHeader("Authorization", equalTo("Bearer test-token"))
-            .withRequestBody(containing("Content-Disposition: form-data; name=\"id\""))
-            .withRequestBody(containing("my-plugin"))
-            .withRequestBody(containing("Content-Disposition: form-data; name=\"version\""))
-            .withRequestBody(containing("2.1.0"))
-            .withRequestBody(containing("Content-Disposition: form-data; name=\"checksum\""))
-            .withRequestBody(containing("sha512:35ab27d09f1bc0d4a73b38fbd020064996fb013e2f92d3dd36bda7364765c229e90e0213fcd90c56fc4c9904e259c482cfaacb22dab327050d7d52229eb1a73c"))
-            .withRequestBody(containing("Content-Disposition: form-data; name=\"provider\""))
-            .withRequestBody(containing("seqera.io")))
-
-        // Verify Step 2: artifact upload
+            .withHeader("Content-Type", equalTo("application/json"))
+            .withRequestBody(containing("\"id\":\"my-plugin\""))
+            .withRequestBody(containing("\"version\":\"2.1.0\""))
+            .withRequestBody(containing("\"checksum\":\"sha512:35ab27d09f1bc0d4a73b38fbd020064996fb013e2f92d3dd36bda7364765c229e90e0213fcd90c56fc4c9904e259c482cfaacb22dab327050d7d52229eb1a73c\""))
+            .withRequestBody(containing("\"provider\":\"seqera.io\"")))
+
+        // Verify Step 2: artifact upload with multipart form data
         wireMockServer.verify(postRequestedFor(urlEqualTo("/api/v1/plugins/release/456/upload"))
             .withHeader("Authorization", equalTo("Bearer test-token"))
             .withRequestBody(containing("Content-Disposition: form-data; name=\"payload\""))