The read-only premise only holds on Python 3.13+ where the full PTB request
MRO is slotted. CI runs 3.11/3.12 where BaseRequest instances still carry a
__dict__, so the unconditional pytest.raises failed. The behavioral half of
the test (subclass re-tag instruments and records) runs everywhere.
Adds a __slots__ request double that has no instance __dict__, mirroring
PTB's HTTPXRequest shape on Python 3.13. The test asserts the old
instance monkey-patch is rejected as read-only and that
_instrument_polling_request instead re-tags the class and still records
getUpdates progress. Fails on the pre-fix adapter with the exact
"'do_request' is read-only" AttributeError from the report.
CI test slices don't install python-telegram-bot (optional dep), causing
a ModuleNotFoundError on collection. Add pytest.importorskip('telegram')
before the PTB imports.
