Fix Streamable HTTP WebClient transport to accept SSE frames without event field

[ZhangDT-sky]

Summary

This PR fixes SSE parsing in WebClientStreamableHttpTransport for Streamable HTTP.

Per SSE semantics, when event: is omitted, the event type defaults to message.
Previously, the transport only accepted frames where event == "message" explicitly, so valid data:-only frames could be dropped.

This could cause JSON-RPC responses (including initialize) to be ignored and lead to startup/timeout failures.

Root Cause

WebClientStreamableHttpTransport.parse(...) only parsed SSE frames when:

• event.event().equals("message")

If event.event() was null (or empty), the frame was ignored.

Changes

• Updated message-event detection in WebClientStreamableHttpTransport to treat all of the following as message events:

1. event == null
2. event == ""
3. event == "message"

• Added integration test:
  WebClientStreamableHttpTransportSseParsingIT#shouldParseSseEventWithoutEventFieldAsMessage

The test serves text/event-stream with only data: (no event:) and verifies the JSON-RPC response is parsed and delivered.

Why This Is Safe

• Scope is limited to SSE event-type filtering in Streamable HTTP WebClient transport.
• Existing explicit event: message behavior is unchanged.
• Fix aligns behavior with standard SSE semantics and improves interoperability with spec-compliant servers.

Verification

• Added dedicated regression test for data:-only SSE frames.
• Local targeted test passed for the new scenario.

Fixes #5780