fix: reinitialize expired streamable sessions

[he-yufeng]

Summary

Fixes #1476.

When a Streamable HTTP server restarts, a client can keep sending POST requests with a stale MCP-Session-Id. The SDK currently reports the server's 404 response as ClientHttpNotImplemented and leaves the expired session in place.

This change handles a 404 from a request that used a session ID as an expired-session signal:

• clear the cached session ID
• let Client run a fresh initialize handshake on the same transport
• retry the original message once with the new session ID

The retry is limited to one attempt so repeated 404s still surface as errors instead of looping.

To verify

• pnpm --filter @modelcontextprotocol/client exec vitest run test/client/streamableHttp.test.ts -t "reinitializes and retries once"
• pnpm --filter @modelcontextprotocol/client exec vitest run test/client/streamableHttp.test.ts
• pnpm --filter @modelcontextprotocol/client typecheck
• pnpm --filter @modelcontextprotocol/client lint
• git diff --check
• pre-push hook: full workspace typecheck, build, and lint

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Latest commit: dc2a285

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