fix: treat omitted SSE event type as default 'message' in WebClientStreamableHttpTransport

[anuragg-saxenaa]

Problem

WebClientStreamableHttpTransport.parse() only processes SSE frames where event.event() equals "message". Per the SSE specification, omitting the event: field means the event type defaults to "message". When Spring/WebFlux decodes such a frame, ServerSentEvent.event() returns null — so the frame was silently dropped.

This caused MCP clients to miss valid JSON-RPC responses from servers that emit bare data:-only SSE frames, resulting in initialization timeouts like:

• Client failed to initialize by explicit API call
• Did not observe any item or terminal signal within ...

This is the Spring AI counterpart of modelcontextprotocol/java-sdk#885 / PR #913, which fixed the HttpClient variant but explicitly noted the WebClient variant needed a separate fix here.

Fix

Accept all three equivalent event types for SSE message frames:

• event == null (field omitted)
• event == "" (field present but empty)
• event == "message" (explicit)

String eventType = event.event();
if (eventType == null || eventType.isEmpty() || MESSAGE_EVENT_TYPE.equals(eventType)) {

Testing

Added testSseFrameWithoutEventTypeIsAccepted() to WebClientStreamableHttpTransportErrorHandlingIT — spins up a local HTTP server that returns an SSE stream with only a data: line (no event: line) and verifies the message handler receives the parsed JSON-RPC message.

Closes #5780