fix: form and type equality bugs

[henryiii]

🤖 AI text below 🤖

Summary

This PR fixes eight bugs and one performance issue in form/type equality and union parameter lookup:

• NumpyForm._is_equal_to (src/awkward/forms/numpyform.py): missing _inner_shape comparison caused NumpyForm("float64", (2,)) == NumpyForm("float64", (3,)) to be True. Fixed with unknown_length-safe per-element comparison.
• RegularForm._is_equal_to (src/awkward/forms/regularform.py) and RegularType._is_equal_to (src/awkward/types/regulartype.py): self._size == other._size raised TypeError when either side was unknown_length (its __eq__ raises on known values). Fixed with the guarded pattern from types/arraytype.py.
• ListType._is_equal_to (src/awkward/types/listtype.py): content comparison used == (which calls is_equal_to with default all_parameters=False), dropping the caller's flag. Fixed by calling _is_equal_to(other._content, all_parameters) directly, matching OptionType.
• UnionMeta.purelist_parameters (src/awkward/_meta/unionmeta.py): unconditional return out inside the for key in keys loop caused only the first key to ever be tried, making __list__-based behavior lookup on unions fail when __record__ was absent. Fixed so the loop continues to the next key when the current one yields no consistent value.
• RecordForm._is_equal_to (src/awkward/forms/recordform.py) and RecordType._is_equal_to (src/awkward/types/recordtype.py): per-field other.content(field) call used list.index → O(n²) for n fields. Fixed by building a field→content dict for other once before the loop.
• ListForm.__init__ (src/awkward/forms/listform.py): error message for invalid stops was a copy-paste of the starts message. Fixed to name and show stops.
• IndexedForm.__init__ (src/awkward/forms/indexedform.py): content=None default always fails validation immediately after; removed the default to make the required-ness explicit.
• ListOffsetForm.__init__ (src/awkward/forms/listoffsetform.py): the only list-like form that did not validate content is a Form; added the check matching sibling forms.

AI assistance

This PR was produced with Claude Code (automated multi-agent review), an AI-assisted workflow. Per CONTRIBUTING.md, this is disclosed here.

Test plan

New regression tests in tests/test_4101_fix_form_type_equality.py cover one focused case per distinct bug:

• NumpyForm inner_shape inequality (different sizes → not equal)
• NumpyForm inner_shape with unknown_length (compatible with any size)
• NumpyForm inner_shape rank mismatch (different number of dimensions → not equal)
• RegularForm/RegularType unknown_length vs. known comparison (no TypeError)
• ListType all_parameters flag propagation to content
• UnionMeta.purelist_parameters second-key resolution (core bug)
• UnionMeta.purelist_parameters inconsistency → None (preserved behaviour)

Existing tests in tests/test_2368_type_is_equal.py, tests/test_2426_is_equal_to.py, tests/test_0914_types_and_forms.py, tests/test_0348_form_keys.py, tests/test_2425_forms_from_type.py all remain green.

prek -a --quiet passes.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.34%. Comparing base (712dac0) to head (09f176d).
⚠️ Report is 2 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/awkward/forms/listoffsetform.py	50.00%	1 Missing ⚠️
src/awkward/forms/recordform.py	90.90%	1 Missing ⚠️

❌ Your patch check has failed because the patch coverage (90.47%) is below the target coverage (98.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

Files with missing lines	Coverage Δ
src/awkward/_meta/unionmeta.py	95.50% <100.00%> (+1.25%)	⬆️
src/awkward/forms/indexedform.py	75.29% <ø> (ø)
src/awkward/forms/listform.py	74.66% <ø> (ø)
src/awkward/forms/numpyform.py	78.74% <100.00%> (ø)
src/awkward/forms/regularform.py	84.28% <100.00%> (+0.22%)	⬆️
src/awkward/types/listtype.py	95.83% <ø> (ø)
src/awkward/types/recordtype.py	76.51% <100.00%> (+0.15%)	⬆️
src/awkward/types/regulartype.py	96.49% <100.00%> (+0.06%)	⬆️
src/awkward/forms/listoffsetform.py	91.17% <50.00%> (-1.25%)	⬇️
src/awkward/forms/recordform.py	94.17% <90.90%> (-0.51%)	⬇️

... and 2 files with indirect coverage changes

[ianna]

@henryiii — looks great! Perhaps worth adding the following to the Agents.md:

nox > python -m pip install 'pylint~=3.3.0'
nox > pylint src