test(desktop): stop flaky gateway SSE stream-count race by retaining response refs

[Astro-Han]

Summary

open-gateway.test.ts › "rejects excess SSE streams" was intermittently failing at assert.equal(activeEventStreams, 10), reading 2–7 instead of 10 under load (#993).

Root cause is in the test, not the gateway. The test opened 10 held-open SSE fetches but kept only each AbortController, discarding the Response. undici aborts a fetch connection when its unread response body is garbage-collected, so under heavier GC (load) the client tore down already-registered streams before the assertion ran. The gateway itself registered all 10 correctly and only removed them on client-initiated req 'close'.

Fix: retain the whole opened stream (controller + response) through the assertions so the bodies stay reachable and the connections stay open.

Closes #993

Verification

• Reproduced on main under 2× CPU load: full-file loop 7/12 failed (2 !== 10, 4 !== 10). Instrumented openSessionEventStream/removeEventClient and confirmed all 10 streams registered, then req close … aborted=true tore them down before the assertion; instrumentation reverted.
• After fix, same load: affected file 20/20 pass; full desktop main suite (2528 tests) 10/10 pass.
• Swept other SSE tests — abuse-contract is a static source scan, runtime/headless parse mock streams — none share this class.

Root cause

undici GC-aborts unconsumed fetch response bodies; the test dropped its Response refs, so GC (load-sensitive) closed registered streams before the count assertion.