fix(com): except pywintypes.com_error raised NameError without pywin32

Python evaluates an `except` clause on the way out of every failed call,
not only the ones it is meant to catch. `pywintypes` is bound only when
`sys.platform == "win32"` and the import succeeded, so on any host
without pywin32 -- all of Linux, and Windows installed without the
`[com]` extra -- an ordinary exception leaving `ComBackend._run` hit that
clause and became `NameError: name 'pywintypes' is not defined`, burying
the real error.

Every Linux CI leg failed on exactly this, in
test_ordinary_errors_are_unchanged. The timeout tests survived only
because they matched an earlier `except`; nothing else in the suite sends
a non-timeout exception through `_run`.

Fixed with a module-level `_COM_ERROR` tuple: `(pywintypes.com_error,)`
where the import worked, `()` where it did not. An empty tuple catches
nothing, which is correct when no com_error type exists to be raised -- a
placeholder Exception subclass would be worse than the bug, silently
swallowing unrelated errors.

Reproduced on Windows before fixing by deleting the module attribute,
which resolves the name the same way the absent import does; that is now
a test, alongside one pinning `_COM_ERROR` as a tuple. Verified through
the locked environment CI uses: 1244 passed, 1 xfailed.

README test-count snapshot 1243 -> 1245.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_com_timeout.py

@@ -117,6 +117,10 @@ async def test_genuine_deadline_still_rebuilds_the_executor(com_backend, monkeyp
     assert await asyncio.wait_for(com_backend._run(lambda: "fresh"), timeout=3) == "fresh"
 
 
+def _raise_value_error():
+    raise ValueError("nope")
+
+
 async def test_ordinary_errors_are_unchanged(com_backend, monkeypatch):
     """Sanity: non-timeout failures never touched the executor and still don't."""
     monkeypatch.setattr(config.settings, "com_call_timeout", 5)
@@ -130,3 +134,36 @@ async def test_ordinary_errors_are_unchanged(com_backend, monkeypatch):
 
     assert com_backend._executor is live
     assert await com_backend._run(lambda x: x * 2, 21) == 42
+
+
+# ── the handler must be inert where pywin32 is absent ───────────────────────
+
+
+async def test_an_ordinary_error_survives_a_host_without_pywin32(com_backend, monkeypatch):
+    """`except pywintypes.com_error` is evaluated on the way out of *every*
+    failed call, not only COM ones.
+
+    `pywintypes` is bound only when `sys.platform == "win32"` and the import
+    succeeded, so on Linux — and on a Windows box that installed without the
+    `[com]` extra — that clause raised `NameError: name 'pywintypes' is not
+    defined` and buried the real exception. Every Linux CI leg failed on
+    exactly this; the timeout tests survived only because they matched an
+    earlier `except`.
+
+    Deleting the module attribute reproduces the absent import faithfully: the
+    clause resolves the name through the module globals either way.
+    """
+    monkeypatch.delattr(cb, "pywintypes", raising=False)
+    monkeypatch.setattr(config.settings, "com_call_timeout", 5)
+
+    with pytest.raises(ValueError, match="nope"):
+        await com_backend._run(_raise_value_error)
+
+
+async def test_the_com_error_clause_is_a_tuple_so_it_can_be_empty():
+    """An empty tuple catches nothing, which is the correct behaviour when no
+    `com_error` type exists to be raised — a placeholder Exception subclass
+    would swallow unrelated errors instead."""
+    assert isinstance(cb._COM_ERROR, tuple)
+    if not cb._COM_IMPORTS_OK:
+        assert cb._COM_ERROR == ()