fix(esc): mid-stream cancellation for OpenAI-compatible + Minimax providers

[ericleepi314]

Summary

• PR fix(esc): cancel streaming API call when abort signal trips #144 added abort-signal-aware streaming for AnthropicProvider, but OpenAICompatibleProvider and MinimaxProvider got only the pre-call fast-path. Users on LiteLLM (OpenAI-compatible) → Claude — the user who reported the lingering 10s ESC pause is on this exact stack — still saw ESC wait the full model latency before the outer query loop's abort check fired.
• Port the response-close listener pattern from fix(esc): cancel streaming API call when abort signal trips #144 to both providers. Mid-stream ESC now unwinds within ~50ms regardless of which provider is in use.

User-reported symptom

After #144 merged, a user testing with LiteLLM → Anthropic Claude Opus 4.7 reported ESC still took ~10s during the model's "Thinking…" phase. The call path went through openai_compatible.py (LiteLLM exposes an OpenAI-compatible API), not anthropic_provider.py, so #144's listener never registered. This PR closes that gap.

Changes

• src/providers/openai_compatible.py — two defenses in chat_stream_response:

  • Response-close listener: registered on abort_signal via add_listener(..., once=True). Calls stream.response.close() to close the underlying httpx socket. The SDK's blocking next-chunk read raises immediately. Handles the user's exact case (long gap between chunks during extended thinking / tool_use generation).
  • In-loop abort check: if abort_signal.aborted: break at the top of each for chunk in stream: iteration. Catches the SDK-prefetched-chunks case where the listener's close lands one iteration late.
  • Register-then-recheck race-safe ordering (matches fix(esc): cancel streaming API call when abort signal trips #144's pattern).
  • Signal-state-authoritative exception translation in the except Exception block.
  • finally block detaches the listener so long-lived controllers don't accumulate listeners.

• src/providers/minimax_provider.py — Minimax uses the anthropic SDK against its compatible endpoint, so it gets the AnthropicProvider treatment (response-close listener; no in-loop check needed because the with ... as stream: only exposes text_stream).

• tests/test_openai_compat_abort_signal.py (new, 6 tests):

  • Pre-abort fast-path skips client.chat.completions.create (leaf-level assert_not_called())
  • Mid-stream close: synthetic 500ms stream + mid-stream trip returns within 1s, verifies stream.response.close() was called
  • Load-bearing in-loop check: passes an on_text_chunk callback, asserts seen == ["first"] not ["first", "second"]. Mutation-tested by deleting the in-loop check and watching the test fail with the exact expected message.
  • Normal-completion regression check
  • abort_signal=None legacy parity
  • Listener detachment after normal completion

• tests/test_minimax_abort_signal.py (new, 4 tests): same shape as the Anthropic tests, with _ensure_client as the fast-path sentinel.

Test plan

• 6 new OpenAI-compat tests pass (1 mutation-verified load-bearing)
• 4 new Minimax tests pass
• 15 total provider abort tests pass (5 Anthropic + 4 Minimax + 6 OpenAI-compat)
• 4419 broader related tests pass
• Critic subagent review: APPROVE (after fixing a stale docstring on this round and a non-load-bearing in-loop test on the prior round)
• Manual: kick off a long prompt on the LiteLLM-proxied stack, press ESC during the model's "Thinking…" phase, should unwind in ~50ms instead of 10s

Follow-up (deferred)

Three-way duplication of the response-close-listener pattern across AnthropicProvider, MinimaxProvider, and OpenAICompatibleProvider. The three contexts differ enough (Anthropic has the watchdog + non-streaming fallback, OpenAI-compat has bare iterator + in-loop check, Minimax has with-block + get_final_message) that premature extraction would either grow the helper to a 4-knob API or leak abstraction. Will file as a separate refactor PR.

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