[mcp] SseMCPTransport.start() never settles when SSE stream closes before endpoint event

## Observed symptom SseMCPTransport.start() never settles when the SSE stream closes before the endpoint event `SseMCPTransport.start()` never settles when the SSE connection closes before the server sends the `endpoint` event. `createMCPClient` awaits `start()`, so client creation hangs with no error and no timeout. A server, proxy, or load balancer that returns `200 text/event-stream` and then closes the stream triggers this. The transport reads `done` from the stream, sees that it is not connected yet, and returns without calling `resolve` or `reject`. ## Environment GitHub issue vercel/ai #21720; state=OPEN ## Trigger / context Public GitHub report in vercel/ai (FRESH_7D). ## Source evidence - https://github.com/vercel/ai/issues/21720 (2026-09-29T10:53:26Z) — evidence_class: PRIMARY_ISSUE_REPORT — GitHub issue #21720 (OPEN) ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:33:42Z - scout_id: C875e344080 ## 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.