fix(server): settle in-flight JSON-mode requests on transport close (v1.x)

[axits-lab]

v1.x backport of #2584. Refs #2559, which notes the item applies to both branches — I offered this on the issue and confirmed the gap is present here before writing anything.

Draft: non-write-access users are capped at one open non-draft PR on this repo and #2582 holds it. Ready for review as-is.

The gap, on this branch

JSON response mode's cleanup in WebStandardStreamableHTTPServerTransport is byte-identical to main's pre-fix code:

cleanup: () => {
    this._streamMapping.delete(streamId);
}

close() runs every stream mapping's cleanup, so a JSON-mode POST whose handler is still running has its map entry dropped while the Promise<Response> returned by handleRequest() is never settled — the HTTP request hangs until the client gives up.

It now resolves with a 503 JSON-RPC error. The success path is untouched: send() resolves before it calls cleanup(), and re-resolving a settled promise is a no-op, so this only fires when nothing was sent.

Scope note

Only the hang is backported. The companion half of #2559 — the _streamMapping leak on completed JSON-mode POSTs — does not reproduce on main (fixed by #2286) and I did not find it here either; send() calls stream.cleanup() on both branches. I left it alone rather than write a no-op change.

Test

Added a JSON response mode lifecycle block driving handleRequest() directly, alongside the existing WebStandardStreamableHTTPServerTransport describes. The handler signals that it has parked via a promise the test awaits, so close() is known to land mid-flight without depending on runner speed, and the hang-detector timer is cleared on the passing path.

Verified it fails without the src change:

× settles an in-flight request when the transport closes mid-handler
  AssertionError: expected true to be false

Verification

• 1640/1640 tests pass across 52 files
• npm run lint and npm run typecheck clean

Changeset included.

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Latest commit: 8e61b9a

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