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

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Latest commit: 31e3308

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commit: 31e3308