fix(server): keep idle GET SSE streams alive

[he-yufeng]

Summary

• Send periodic SSE comment frames on idle standalone GET streams.
• Clear the timer when the stream is cancelled or closed by transport cleanup.
• Add a fake-timer regression test for the first keep-alive frame.

Fixes #1211.

Validation

• pnpm --filter @modelcontextprotocol/server exec vitest run test/server/streamableHttp.test.ts -t "keep-alive"
• pnpm --filter @modelcontextprotocol/server exec vitest run test/server/streamableHttp.test.ts
• pnpm --filter @modelcontextprotocol/server run typecheck
• pnpm --filter @modelcontextprotocol/server run lint
• git diff --check

[changeset-bot]

🦋 Changeset detected

Latest commit: 7f08da4

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@modelcontextprotocol/server	Patch

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commit: 7f08da4

[lppedd]

This seems to be working in prod. Patched my own library with the changes and all good.

[he-yufeng]

Rebased onto current main, added the server patch changeset, and force-pushed 7f08da4. Local validation: changeset status shows @modelcontextprotocol/server patch; server streamableHttp.test.ts passed with 41 tests; server typecheck and lint passed; git diff --check passed.

[he-yufeng]

Correction: the previous comment was posted after my first push attempt failed in the local pre-push hook with a transient pnpm workspace-state JSON parse error. I reran the push; pre-push typecheck, build, and lint all passed, and 7f08da4 is now pushed.