fix: copy request builder to prevent side effects in HTTP transports (#333)

- Add .copy() call before modifying requestBuilder in FlowSseClient
- Add .copy() call before modifying requestBuilder in HttpClientSseClientTransport
- Prevents unintended mutations of shared builder instances

Author: JooKS-me

mcp/src/main/java/io/modelcontextprotocol/client/transport/FlowSseClient.java

@@ -121,7 +121,8 @@ public FlowSseClient(HttpClient httpClient, HttpRequest.Builder requestBuilder)
 	 * @throws RuntimeException if the connection fails with a non-200 status code
 	 */
 	public void subscribe(String url, SseEventHandler eventHandler) {
-		HttpRequest request = this.requestBuilder.uri(URI.create(url))
+		HttpRequest request = this.requestBuilder.copy()
+			.uri(URI.create(url))
 			.header("Accept", "text/event-stream")
 			.header("Cache-Control", "no-cache")
 			.GET()

mcp/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java

@@ -415,7 +415,8 @@ public Mono<Void> sendMessage(JSONRPCMessage message) {
 		try {
 			String jsonText = this.objectMapper.writeValueAsString(message);
 			URI requestUri = Utils.resolveUri(baseUri, endpoint);
-			HttpRequest request = this.requestBuilder.uri(requestUri)
+			HttpRequest request = this.requestBuilder.copy()
+				.uri(requestUri)
 				.POST(HttpRequest.BodyPublishers.ofString(jsonText))
 				.build();