Update Accept header for SSE requests to use application/json, text/event-stream

Author: kpavlov

kotlin-sdk-client/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/StreamableHttpClientTransport.kt

@@ -228,7 +228,8 @@ public class StreamableHttpClientTransport(
             ) {
                 method = HttpMethod.Get
                 applyCommonHeaders(this)
-                accept(ContentType.Text.EventStream)
+                // sseSession will add ContentType.Text.EventStream automatically
+                accept(ContentType.Application.Json)
                 (resumptionToken ?: lastEventId)?.let { headers.append(MCP_RESUMPTION_TOKEN_HEADER, it) }
                 requestBuilder()
             }

kotlin-sdk-client/src/jvmTest/kotlin/io/modelcontextprotocol/kotlin/sdk/client/MockMcp.kt

@@ -67,7 +67,7 @@ internal class MockMcp(verbose: Boolean = false) {
         mokksy.get(name = "MCP GETs", requestType = Any::class) {
             path("/mcp")
             containsHeader("Mcp-Session-Id", sessionId)
-            containsHeader("Accept", "text/event-stream,text/event-stream") // todo: why 2 times?
+            containsHeader("Accept", "application/json,text/event-stream")
             containsHeader("Cache-Control", "no-store")
         } respondsWithSseStream {
             headers += "Mcp-Session-Id" to sessionId