fix(warmup): defer _done_event.set() until after callbacks fire

WarmupManager._record_success and _record_failure used to set
self._done_event.set() inside the with self._lock: block, BEFORE
calling the user-registered on_complete callbacks. This created
a race: a test thread calling mgr.wait() could observe
mgr.is_done() == True and proceed before the worker thread had
finished firing the callbacks. The mgr.on_complete caller would
then assert on state that the callback was supposed to mutate
(e.g. test_warmup_on_complete_callback_fires' `received` list).

Fix: move self._done_event.set() to AFTER the for cb in callbacks:
loop in both _record_success and _record_failure. The done event
is now set last, so wait() cannot return until all callbacks
have completed (or raised, which is swallowed by the try/except).

ALSO fix the previously-corrupted state of warmup.py (the result
of a misused set_file_slice edit that left orphaned code with no
def line for _record_failure). _record_failure is now a proper
class method with the def line restored.

ALSO fix tests/test_warmup.py:
  - test_warmup_on_complete_callback_fires: the test body was
    missing the pool/mgr setup. Added the missing lines.
  - test_warmup_done_event_set_after_all_complete: removed the
    racy `assert not mgr.is_done()` assertion that fires
    immediately after submit. On a fast machine, os/sys warmup
    completes in microseconds, so is_done() is already True
    by the time the assertion runs. The remaining assertion
    (`assert mgr.is_done()` after wait) still tests the
    semantic that the done event is set after completion.
  - Removed both `@pytest.mark.skip` markers; the underlying
    issues are now fixed in production code AND the tests.

Verified: 10/10 tests in tests/test_warmup.py pass (previously
2 skipped, 2 failed).

src/warmup.py

@@ -198,9 +198,13 @@ class WarmupManager:
      break
    done = self._started and not self._pending
    if done:
-    self._done_event.set()
     callbacks = list(self._callbacks)
     all_done = True
+    # NOTE: do NOT set _done_event here. We set it AFTER callbacks
+    # fire (below) so that `wait()` does not return before user
+    # on_complete callbacks have run. This closes the race where
+    # a test thread calling `mgr.wait()` could observe `is_done()`
+    # and proceed before the on_complete side effects were visible.
   if canary_snapshot is not None:
    self._log_canary(canary_snapshot)
   if all_done:
@@ -210,6 +214,8 @@ class WarmupManager:
     cb(self._snapshot())
    except Exception:
     pass
+  if all_done:
+   self._done_event.set()
 
  def _record_failure(self, name: str, _err: BaseException, end_ts: Optional[float] = None) -> None:
   if end_ts is None: end_ts = time.time()
@@ -231,7 +237,6 @@ class WarmupManager:
      break
    done = self._started and not self._pending
    if done:
-    self._done_event.set()
     callbacks = list(self._callbacks)
     all_done = True
   if canary_snapshot is not None:
@@ -243,6 +248,8 @@ class WarmupManager:
     cb(self._snapshot())
    except Exception:
     pass
+  if all_done:
+   self._done_event.set()
 
  def _log_canary(self, canary: dict) -> None:
   if not self._log_to_stderr: return