Deliver SSE terminal frames after a mailbox overflow

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Problem

sse_mailbox_push returns false as soon as overflowed is set. force_deliver is supposed to evict oldest events so a terminal SSE envelope (env.terminal on request replay and A2A resubscribe) still lands and the stream can close cleanly.

The overflow check ran first. A full mailbox that rejected one more live event therefore also dropped the subsequent done / task-status frame. The subscriber drain loop in handle_request_events / A2A resubscribe then wrote slow_consumer and exited without the real terminal. The existing unit test only covered force-deliver while the mailbox was full but not yet overflowed.

Fix

Honor force_deliver after overflow: evict oldest items if still at cap, push the terminal envelope, and clear overflowed so the drain loop writes that frame instead of aborting.

Non-terminal pushes after overflow still reject.

Tests

unit_sse_mailbox — overflow then force-deliver: terminal is at the back, overflowed is cleared, dropped live event never queued. All 3 cases / 6163 assertions locally (including ASan). Consumer-loop probe matching the API drain also delivered done instead of slow_consumer.

Independently mergeable against main (0456350). Code hunks (include/sse_mailbox.h, tests/test_sse_mailbox.cpp) do not overlap #321–#337. git merge-tree --write-tree auto-merges those siblings, including CHANGELOG [Unreleased].

  

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Note

Low Risk
Small, targeted change to bounded SSE buffering with a new regression test; affects slow-client stream teardown only, not normal event delivery.

Overview
Fixes a bug where slow SSE subscribers could miss the real stream ending after the mailbox had already overflowed.

sse_mailbox_push no longer rejects every push once overflowed is set. With force_deliver=true (terminal done / task-status envelopes), it still evicts oldest buffered events if needed, enqueues the terminal frame, and clears overflowed so the drain loop emits that frame instead of only slow_consumer. Non-terminal pushes after overflow still fail as before.

Adds a unit test for overflow-then-force-deliver and documents the fix in CHANGELOG.

^{Reviewed by Cursor Bugbot for commit 46172fa. Bugbot is set up for automated code reviews on this repo. Configure here.}

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