Fix sticky unresolvable __all__ state

[tkim602]

Summary

Fixes #4563

keep an unresolvable __all__ state from being reset by later mutations and preserve DunderAllKind::Unresolvable across subsequent append, extend, and remove calls so star imports continue to use the fallback behavior.

Test Plan

Added regression coverage for:

• keeping DunderAllKind::Unresolvable sticky across later __all__ mutations
• preserving star-import fallback behavior after an unresolvable append

Ran:

cargo test test_all_unresolvable_is_sticky_across_mutations -- --nocapture
cargo test test_all_ -- --nocapture
cargo fmt --check
git diff --check

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