[ty] Add membership narrowing ecosystem regression (#26991)
## Summary
Adds a regression test for the [ibis ecosystem hit surfaced by
#26988](https://github.com/astral-sh/ruff/pull/26988#issuecomment-5017218003).
Membership narrowing previously replaced an explicitly annotated literal
union with promotable literals from the tuple, causing an inferred
instance attribute to widen to `str` when read from another method.
The test ensures that the literal union is preserved across the
assignment and subsequent return.
