Resolve field aliases before calling field_transformer

[veeceey]

Summary

Fixes #1479

• Moves default alias resolution (e.g., stripping leading underscores from _private to private) to before the field_transformer callback, so transformers receive fully populated Attribute objects with usable alias values instead of None.
• Adds a second alias resolution pass after the transformer to handle any new fields the transformer may have added without setting an alias.
• Updates Attribute.evolve() to automatically update the alias when the name changes, if the alias was auto-generated (matches the default for the old name). Explicit aliases are preserved.

Before (broken)

def transformer(cls, fields):
    for f in fields:
        print(f.alias)  # None for auto-generated aliases
    return fields

After (fixed)

def transformer(cls, fields):
    for f in fields:
        print(f.alias)  # 'private' for _private, 'my_alias' for explicit
    return fields

Test plan

• Added test_hook_alias_available - verifies transformer sees resolved aliases
• Added test_hook_evolve_name_updates_auto_alias - verifies evolve(name=...) updates auto-generated alias
• Added test_hook_evolve_name_keeps_explicit_alias - verifies evolve(name=...) preserves explicit alias
• Updated 2 existing test expectations where error messages now show the resolved alias instead of None
• Full test suite passes: 1367 passed, 6 skipped, 2 xfailed

🤖 Generated with Claude Code