Python: fix AutoFormat crash on import ... as ...

[knutwannheden]

Motivation

AutoFormat crashed on any Python file containing import X as Y or from X import Y as Z, surfacing as AttributeError: 'Identifier' object has no attribute 'before' deep in SpacesVisitor. Discovered via flagship recipe alerts while running UnwrapElseAfterReturn on real sources.

Root cause is a subtle mismatch between public and private fields on padded LST nodes. Import._alias is typed Optional[JLeftPadded[Identifier]], while the public alias property unwraps to the bare Identifier. The previous code did:

imp = imp.replace(alias=space_before(imp.alias, True))
imp = imp.padding.replace(alias=space_before_left_padded(imp.padding.alias, True))

replace_if_changed maps the public kwarg alias to the private field _alias without type-checking the value, so the first line clobbered the JLeftPadded wrapper with a raw Identifier. The second line then read imp.padding.alias back as a bare Identifier and crashed on .before.

Summary

• SpacesVisitor.visit_import now applies both spacing adjustments to the JLeftPadded wrapper (using space_before_left_padded for the space before as, and space_before_left_padded_element for the space before the alias identifier) before a single padding.replace.
• Added a regression test covering both import X as Y and from X import Y as Z with non-normalized spacing.

Test plan

• New test_import_with_alias_normalizes_spacing fails on trunk with the reported AttributeError and passes after the fix
• Full format + import parse suite: pytest tests/python/all/format/ tests/python/all/tree/import_test.py tests/python/format/ — 181 passed
• Broader smoke: pytest tests/ --ignore=tests/rpc — 1157 passed, 6 skipped