Fix httpRequestCustomizer usage in HttpClientStreamableHttpTransport

- Closes #458

Signed-off-by: Daniel Garnier-Moiroux <git@garnier.wf>

Author: Kehrlann

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

@@ -400,7 +400,7 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage sentMessage) {
 					.header("Cache-Control", "no-cache")
 					.header(HttpHeaders.PROTOCOL_VERSION, MCP_PROTOCOL_VERSION)
 					.POST(HttpRequest.BodyPublishers.ofString(jsonBody));
-				return Mono.from(this.httpRequestCustomizer.customize(builder, "GET", uri, jsonBody));
+				return Mono.from(this.httpRequestCustomizer.customize(builder, "POST", uri, jsonBody));
 			}).flatMapMany(requestBuilder -> Flux.<ResponseEvent>create(responseEventSink -> {
 
 				// Create the async request with proper body subscriber selection

mcp/src/test/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransportEmptyJsonResponseTest.java

@@ -84,7 +84,7 @@ void testNotificationInitialized() throws URISyntaxException {
 		StepVerifier.create(transport.sendMessage(testMessage)).verifyComplete();
 
 		// Verify the customizer was called
-		verify(mockRequestCustomizer, atLeastOnce()).customize(any(), eq("GET"), eq(uri), eq(
+		verify(mockRequestCustomizer, atLeastOnce()).customize(any(), eq("POST"), eq(uri), eq(
 				"{\"jsonrpc\":\"2.0\",\"method\":\"initialize\",\"id\":\"test-id\",\"params\":{\"protocolVersion\":\"2025-03-26\",\"capabilities\":{\"roots\":{\"listChanged\":true}},\"clientInfo\":{\"name\":\"Spring AI MCP Client\",\"version\":\"0.3.1\"}}}"));
 
 	}

mcp/src/test/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransportTest.java

@@ -80,7 +80,7 @@ void testRequestCustomizer() throws URISyntaxException {
 			StepVerifier.create(t.sendMessage(testMessage)).verifyComplete();
 
 			// Verify the customizer was called
-			verify(mockRequestCustomizer, atLeastOnce()).customize(any(), eq("GET"), eq(uri), eq(
+			verify(mockRequestCustomizer, atLeastOnce()).customize(any(), eq("POST"), eq(uri), eq(
 					"{\"jsonrpc\":\"2.0\",\"method\":\"initialize\",\"id\":\"test-id\",\"params\":{\"protocolVersion\":\"2025-03-26\",\"capabilities\":{\"roots\":{\"listChanged\":true}},\"clientInfo\":{\"name\":\"Spring AI MCP Client\",\"version\":\"0.3.1\"}}}"));
 		});
 	}
@@ -107,7 +107,7 @@ void testAsyncRequestCustomizer() throws URISyntaxException {
 			StepVerifier.create(t.sendMessage(testMessage)).verifyComplete();
 
 			// Verify the customizer was called
-			verify(mockRequestCustomizer, atLeastOnce()).customize(any(), eq("GET"), eq(uri), eq(
+			verify(mockRequestCustomizer, atLeastOnce()).customize(any(), eq("POST"), eq(uri), eq(
 					"{\"jsonrpc\":\"2.0\",\"method\":\"initialize\",\"id\":\"test-id\",\"params\":{\"protocolVersion\":\"2025-03-26\",\"capabilities\":{\"roots\":{\"listChanged\":true}},\"clientInfo\":{\"name\":\"Spring AI MCP Client\",\"version\":\"0.3.1\"}}}"));
 		});
 	}