codex: tighten pyi AST parser helpers

[saidctb]

Summary

Test plan

• PYTHONPATH=. pytest -q

Merge checklist

• All required checks are green (tests + parser-reference-guard).
• If parser Fortran data or goldens changed, the PR explains why.
• If using ignore-parser-reference-guard, the PR explains why it is safe.

[codecov]

Codecov Report

❌ Patch coverage is 91.52542% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
semantics/pyi_parser.py	91.52%	2 Missing and 3 partials ⚠️

Flag	Coverage Δ
py312	96.64% <91.52%> (-0.11%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
semantics/pyi_parser.py	96.14% <91.52%> (-0.90%)	⬇️

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