feat: declare the attribute contracts the cancel and test mixins rely on

[blaipr]

HasCancel and HasTest read self.page and self.resource, which the CustomAction they are mixed into supplies. AssociationMixin, a few classes down the same file, has carried the annotations describing exactly that since #50 and #51. These two arrived in #62 and #65 without them, so the type checker reported seven unresolved attributes for a contract the file already knew how to state.

    # Supplied by the CustomAction this is mixed into. Annotations rather than
    # assignments: they describe the contract without creating class attributes
    # that would shadow the real ones.
    page: 'api.pages.Page'
    resource: str

Annotations rather than assignments, for the reason the comment on AssociationMixin already gives: an assignment would create a class attribute that shadows the real one at runtime.

Verification

• ty goes from 35 diagnostics to 28, which is the seven these two classes produced and nothing else. The one finding left in this file, the Launchable.add_arguments override on line 260, predates feat: add the cancel action to the CLI #62 and feat: add the test action to the CLI #65 and is not touched here.
• Neither page nor resource appears in vars() on either class, and both appear in __annotations__. That is the property the comment claims, checked rather than assumed.
• The action registry still resolves all six cancel entries and all three test entries.
• ruff format --check, ruff check and the unit suite (366 passing) are unchanged.

Scope

This is the uncontroversial part of the follow-up #42 set up. The remaining 28 diagnostics are genuine annotation gaps in older code, each needing a small decision of its own: a TypedDict for the heterogeneous inventory structure in ascender/inventory.py, the self.__class__(...) constructor calls in pages/page.py, and the override signatures in pages/notifications.py and cli/custom.py:260. None of them is a runtime bug, checked individually. They are not in this pull request.

[ciq-it-service-account]

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