fix: retain streamable HTTP session on response write failure

[DragonFSKY]

Fixes #952.

Summary

• Keep the MCP session registered when a request-specific streamable HTTP response write fails.
• Continue completing the failed request async context without treating the write failure as an explicit session termination.
• Add a regression test that simulates PrintWriter.checkError() after an SSE response write and verifies a subsequent request using the same session id does not return 404.

Verification

• ./mvnw -pl mcp-test -am -Dtest=HttpServletStreamableSessionFailureTests -Dsurefire.failIfNoSpecifiedTests=false test
• ./mvnw -pl mcp-test -am -Dtest=HttpServletStreamableSessionFailureTests,HttpServletStreamableSyncServerTests,HttpServletStreamableAsyncServerTests,HttpServletStreamableIntegrationTests -Dsurefire.failIfNoSpecifiedTests=false test
• ./mvnw -pl mcp-core,mcp-test -am spring-javaformat:validate
• git diff --check