…ncel

PRs #144 and #145 added abort-signal-aware streaming for three
providers (Anthropic, Minimax, OpenAI-compatible). The pattern across
all three is structurally identical — pre-call fast-path, register-
then-recheck listener closing the stream's response on abort, signal-
state-authoritative exception translation, listener detachment, post-
stream recheck — but the bookkeeping was inlined and triplicated.
Adding a fourth provider would have meant a fourth copy.

Extract the pattern into ``src/providers/_stream_abort.py``:

* ``StreamAbortGuard(abort_signal)`` — one per call. ``abort_signal=None``
  makes every method a no-op so providers can use it unconditionally.
* ``raise_if_pre_aborted()`` — pre-call fast-path raising ``AbortError``.
* ``raise_if_post_aborted()`` — same check at a different boundary.
* ``aborted`` property — cheap in-loop check (OpenAI-compat path).
* ``reraise_if_aborted(original_exc)`` — translate SDK exceptions to
  ``AbortError`` via ``raise ... from``; no-op if signal didn't fire.
* ``with guard.attach(stream):`` — register a listener that closes
  ``stream.response`` on abort; detach in ``__exit__``. ``__exit__``
  ALSO closes the response if ``signal.aborted`` is True at exit, as
  a safety net for the race window where ``AbortSignal._fire``
  snapshots the listener list, the consumer thread observes
  ``aborted=True``, breaks out, detaches the listener, and the
  snapshot's iteration then runs against an empty list — that path
  would otherwise leak the underlying httpx response open. The close
  is idempotent on httpx (``if not self.is_closed`` guard) so the
  double-fire path is safe.

Provider line counts:

  anthropic_provider.py    -88 lines
  minimax_provider.py      -73 lines
  openai_compatible.py    -212 lines
  _stream_abort.py        +211 (mostly docstring)

Net: providers shrink from 374 lines of abort bookkeeping to 110
(SDK-specific iteration shape only). The provider-level tests from
#144 and #145 (15 tests across three files) pass unmodified, proving
behavior preservation end-to-end.

Seventeen new unit tests in ``tests/test_stream_abort_guard.py`` pin
the helper's contract directly: pre/post fast-paths, ``aborted``
property semantics, ``attach`` lifecycle (register, detach, race-
recovery via register-then-recheck), close-failure tolerance, no-
response-attribute graceful degradation, ``reraise_if_aborted``
translation + cause chaining + no-op, and — critically — the
``__exit__`` close-on-abort safety net. The safety-net test
mutation-verified: removing the ``__exit__`` close branch makes the
test fail with the exact expected assertion.

The OpenAI-compat in-loop test gains a ``stream.response.close.called``
assertion to pin the close at the provider level too.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>