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>

README.md

@@ -16,7 +16,7 @@ refine quality issues inside transactions, and deliver hashed artifacts with
 validation evidence.
 
 > **v1.5 release snapshot:** 154 tools · 6 resources · 5 prompt templates ·
-> 1243 collected tests. Runtime discovery through `system_about` is authoritative.
+> 1245 collected tests. Runtime discovery through `system_about` is authoritative.
 
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