Replace ty: ignore with proper type declarations for inheritance patterns

[rahuldevikar]

Several base classes used attributes that are only defined in subclasses (via mixins). ty couldn't see those attributes from the base class perspective, so we had ty: ignore[unresolved-attribute] and ty: ignore[not-iterable] comments.

Type checkers analyze each class in isolation. When Describe provides exe but Creator accesses self.exe, the checker doesn't see the connection because they're only combined at the concrete class level. Our fix was to add property stubs in the base class

Extension work of: #3029

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• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation