fix(server): restore onclose after modern exchanges

[vjymisal0]

Summary

Fixes #2607.

When a createMcpHandler factory returns the same McpServer instance for multiple modern requests, the handler temporarily wraps server.onclose for in-flight tracking. The wrapper removed the server from the in-flight set but did not restore the previous onclose, so reused servers accumulated a nested wrapper chain across requests.

This restores the previous onclose before invoking it, so each completed exchange leaves a reused server with its original close callback instead of another wrapper layer.

Tests

• pnpm --filter @modelcontextprotocol/server exec vitest run test/server/createMcpHandler.test.ts -t "restores a reused server onclose"
• pnpm --filter @modelcontextprotocol/server exec vitest run test/server/createMcpHandler.test.ts
• pnpm exec prettier --check packages/server/src/server/createMcpHandler.ts packages/server/test/server/createMcpHandler.test.ts
• pnpm --dir packages/server exec eslint src/server/createMcpHandler.ts
• pnpm --filter @modelcontextprotocol/server run typecheck

Note: pnpm --filter @modelcontextprotocol/server run check currently reaches Prettier warnings across many existing package files unrelated to this patch, so I also ran targeted Prettier/ESLint on the touched files.

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Latest commit: 94d37c4

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