Fix #278: bound the OpenAI-compatible ESC-cancel queue

[ericleepi314]

Closes #278

Stacked on #308 (← #307 ← #306 ← #305 ← #304). Merge in order; GitHub retargets automatically.

Summary

The ESC-cancel worker-thread pattern in OpenAICompatibleProvider.chat_stream_response used an unbounded queue.Queue — flagged as a follow-up in PR #148. A pathological proxy that keeps sending bytes after ESC (and never closes the SDK iterator) accumulated megabytes while the consumer had already raised AbortError and left.

• Queue bounded at 64.
• Worker-side drop: _put_or_drop_on_abort stops the worker from reading the stream at all once the abort trips — the queue drains, the consumer hits the Empty tick, and AbortError raises within ~100ms even against a firehose.
• consumer_gone event (from critic review): without it, the bounded queue introduced a new failure mode the unbounded one didn't have — a consumer that unwinds for a non-abort reason (e.g. on_text_chunk raising) left the worker retrying a full queue forever, pinning the httpx connection open. The consumer scope sets the event via contextlib.ExitStack on every exit path; the worker bails within one 0.25s poll.
• A genuine stream error that loses the race against ESC is logged at debug instead of vanishing.
• Consumer loop deliberately unchanged: chunks received before the abort are still processed (pinned by test_in_loop_check_catches_abort_between_chunks); the worker-side drop is what guarantees nothing arrives after.

Test plan

• New: firehose stream (ignores close(), yields forever) → AbortError <2s and the yield counter settles (worker stopped reading); consumer-crash (callback raises ValueError) → error propagates with class intact and the worker is released; 150-chunk backpressure happy path → exact content/order through the bounded queue
• All 11 tests in the file + 71 provider-abort tests pass
• Full suite on the stack: 7797 passed, 0 failed, 5 skipped
• Critic review loop: APPROVE after 1 revision round (the consumer-crash livelock was found there, repro'd empirically)

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