Flush headers for idle Streamable HTTP GET requests

[karthiksenv]

An idle Streamable HTTP GET currently sends neither status nor headers until the first SSE event, causing clients to time out when keep-alive is disabled. Flush the servlet response after registering the listening stream so clients receive HTTP 200 and Content-Type: text/event-stream immediately.

Adds an embedded-Tomcat regression test with keep-alive disabled. The test timed out before the fix and passes with it.

Fixes #1155.

Validation:

• Regression test: passed after reproducing the failure before the fix.
• HttpServletStreamableIntegrationTests: all 54 tests passed.
• mvnw.cmd -pl mcp-core,mcp-test -am test: core and Jackson modules passed; mcp-test ran 823 tests with 13 errors, including unavailable Docker/dependent initialization errors and a stdio timeout. The broader suite is not fully green in this environment.
• Tested with Java 17 and embedded Tomcat 11.0.2; the reporter's exact JDK/Tomcat versions were not reproduced.
• Formatting validation and git diff --check passed.

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