test: cover AST analyzer edge cases

[charles-openclaw]

Description

Adds focused AST analyzer edge-case coverage for existing behavior:

• keyword-only mutable defaults are detected
• unreachable code in async functions is detected
• used from ... import ... as ... aliases are not reported as unused
• star imports are skipped by unused-import detection

Related Issue

Fixes #466

Type of change

• Bug fix
• New feature / enhancement
• Documentation update
• Test addition
• Refactor

Checklist

• I have read CONTRIBUTING.md
• My branch is up to date with main
• I have run pytest -v and all tests pass
• I have not introduced duplicate issues or features
• My PR title follows the format: feat/fix/docs/test: short description
• I have added tests for new features (Level 2 and 3 issues)
• No hardcoded secrets or API keys in my code
• This PR is linked to a GSSoC 2026 issue

Screenshots (if frontend change)

N/A - backend tests only.

Test evidence

cd backend
PATH="$HOME/.local/bin:$PATH" uv run --with-requirements requirements.txt pytest -v
# 376 passed, 2 warnings in 7.34s

Additional focused check before the full suite:

PATH="$HOME/.local/bin:$PATH" uv run --with-requirements backend/requirements.txt pytest backend/tests/test_ast_analyzer.py backend/tests/test_python_ast_analyzer.py -q
# 51 passed in 0.11s

Also ran:

git diff --check