fix(server): replay request SSE responses after closeSSEStream

[fallintoplace]

Summary

• persist request-scoped SSE replay events even after closeSSEStream() detaches the live controller
• avoid throwing when a terminal SSE response arrives during the reconnect window and replay is still possible
• add regression coverage in the shared server transport tests and the Node Streamable HTTP wrapper tests

Root cause

closeSSEStream() intentionally removes the active response stream so a long-running request can continue while the client reconnects. The request-scoped send() path still treated the live controller as a prerequisite for storing replay events, so a terminal response produced during that reconnect window could not be replayed and instead failed with No connection established for request ID.

Validation

• npx -y pnpm@10.26.1 --filter @modelcontextprotocol/server test -- test/server/streamableHttp.test.ts
• npx -y pnpm@10.26.1 --filter @modelcontextprotocol/node test -- test/streamableHttp.test.ts
• npx -y pnpm@10.26.1 --filter @modelcontextprotocol/server typecheck
• npx -y pnpm@10.26.1 --filter @modelcontextprotocol/node typecheck
• npx -y pnpm@10.26.1 --filter @modelcontextprotocol/server lint
• npx -y pnpm@10.26.1 --filter @modelcontextprotocol/node lint
• ASDF_NODEJS_VERSION=24.15.0 git push -u origin fix-close-sse-request-replay (repo pre-push hook: build, typecheck, lint)

Fixes #2151

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