Fix dependency group malformed parse caching

[r266-tech]

Refs #1239

Summary

• reject non-string include-group values before constructing a dependency-group include
• avoid storing parsed dependency groups when parsing collected errors, so repeated lookups keep reporting malformed data
• add regression coverage for repeated malformed lookup on the same resolver

Tests

• uv run pytest tests/test_dependency_groups.py -q
• uv run ruff check src/packaging/dependency_groups.py tests/test_dependency_groups.py

[r266-tech]

Done in 3cecdcf — switched the include-group type error to the msg = ...; errors.error(TypeError(msg)) form.

[r266-tech]

Switched the error-path to return () per @sirosen — agreed it reads more clearly that no meaningful result is produced (the value is discarded anyway, since lookup raises when errors.errors is non-empty). @henryiii thanks for the latitude. The malformed-parse caching fix and its regression test are unchanged.