[mcp_server] [Bug] SseServerTransport.close() fails with active sessions and leaks the HTTP server port

## Observed symptom [Bug] SseServerTransport.close() fails with active sessions and leaks the HTTP server port Description SseServerTransport.close() fails whenever at least one SSE session is open. The HTTP server is never closed, so the port stays bound for the lifetime of the process, and the error is unrecoverable by callers. `// lib/src/transport/transport.dart:519` ## Environment GitHub app-appplayer/mcp_server #5 ## Trigger / context Public GitHub report in app-appplayer/mcp_server (FRESH_7D). ## Source evidence - https://github.com/app-appplayer/mcp_server/issues/5 (2026-09-30T06:26:33Z) — evidence_class: PRIMARY_ISSUE_REPORT — #5 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T10:10:25.435874Z - scout_id: C70f3b85ec0 ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome. Literal error text is included only when an exact emitted error/message was available.