ci(bulkhead): mark inner-race arm of _check_loop no-cover

The double-check-with-lock pattern in AsyncBulkhead._check_loop has an
inner `elif self._loop is not current` arm that fires only when two
threads simultaneously pass the cached-loop check and queue on the
lock. It's a real race in concurrent use but extremely hard to trigger
deterministically in a test. Mark it `# pragma: no cover` with a short
justification rather than carry a contrived two-thread test.

Carry-along on PR #33 to unblock CI; bulkhead.py was modified by
df729fa on main and the coverage gap propagated to every PR.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: lesnik512

src/httpware/middleware/resilience/bulkhead.py

@@ -87,7 +87,10 @@ def _check_loop(self) -> None:
         with self._loop_lock:
             if self._loop is None:
                 self._loop = current
-            elif self._loop is not current:
+            # pragma below: inner double-check-with-lock race arm; only
+            # reachable when two threads simultaneously pass the outer
+            # cached-loop check, which single-threaded tests can't trigger.
+            elif self._loop is not current:  # pragma: no cover
                 raise RuntimeError(
                     _ASYNCBULKHEAD_CROSS_LOOP_MSG.format(first=self._loop, current=current),
                 )