[Epic] SSE streaming hardening

Tracking epic for SSE/streaming response bugs: deferred teardown leaking listeners/sockets, and a finite async generator hanging the response with an uncaughtException. Related: #1290 (unbounded live-tail queue) is tracked under #1566 (unbounded-queue/DoS family) instead, since it's a duplicate of that cluster's canonical issue.

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_Filed by Claude (Opus 4.8), issue-backlog triage & grouping pass._

This is a broad SSE/streaming hardening epic with no files or tests named. Start by reproducing the deferred-teardown leak and the finite async-generator hang, then trace the response handling involved. Done means listeners and sockets are released, finite generators complete without hanging or uncaughtException, and the unbounded live-tail queue remains out of scope under #1566.

Written by the indexing model from the issue text.