fix(server): commit the SSE response before the first event on GET - #1194

A GET to the streamable endpoint sets the SSE headers, starts async, and then waits for the session to write something. Nothing commits the response until that first event, so a client that waits for the response before reading blocks: Node's fetch (undici) fails with UND_ERR_HEADERS_TIMEOUT after its 300 s default, mcp-remote retries twice and gives up, and the server is left without a listening stream, which also breaks server-initiated requests (Stream unavailable for session).

response.flushBuffer() right after startAsync() commits the status line and headers. That is the same call HttpServletMcpTransport already makes for the stateless transport.

Verification (Windows, JDK 21):

• New HttpServletStreamableIntegrationTests#listeningStreamCommitsResponseBeforeFirstEvent opens a listening stream against a provider without keep-alive, so nothing will ever be written to it. It asserts the client gets 200 plus Content-Type: text/event-stream within 5 s. Before the fix the same test times out in sendAsync(...).get(...); with the flush it returns in ~2 s.
• ./mvnw -pl mcp-test -am test -Dtest='HttpServletStreamableIntegrationTests#listeningStreamCommitsResponseBeforeFirstEvent' passes.
• spring-javaformat:apply was run, the build's validate phase is clean.

Fixes #1155