fix(parser): track keyword argument callback references

[lamchun1110]

Summary

• extract Python callback references from keyword_argument values
• reuse the existing known-definition/import guard to avoid noisy edges
• add regression coverage for set_defaults(func=handler) and dead-code detection

Fixes #840

Testing

• uv run pytest tests/test_parser.py -q --disable-warnings --maxfail=1 (163 passed)
• uv run pytest tests/ --tb=short -q (2399 passed, 5 skipped, 2 xpassed)
• uv run ruff check code_review_graph/parser.py
• uv run mypy code_review_graph/ --ignore-missing-imports --no-strict-optional

[github-actions]

code-review-graph review

Overall risk: 0.30 (LOW) — 7 changed function(s)/class(es), 0 affected flow(s), 4 test gap(s)

Risk-scored changes

Risk	Level	Symbol	Location	Tested
0.30	low	code_review_graph/parser.py::CodeParser	code_review_graph/parser.py:2404	yes
0.30	low	tests/fixtures/sample_callback_refs.py::keyword_callback	tests/fixtures/sample_callback_refs.py:22	no
0.30	low	tests/fixtures/sample_callback_refs.py::register_keyword_callback	tests/fixtures/sample_callback_refs.py:42	no
0.25	low	code_review_graph/parser.py::CodeParser._ref_from_arguments	code_review_graph/parser.py:12184	no
0.05	low	tests/test_parser.py::TestCodeParser	tests/test_parser.py:13	no
0.05	low	tests/test_parser.py::TestCodeParser.test_python_keyword_callback_reference_emitted	tests/test_parser.py:845	(test)
0.05	low	tests/test_parser.py::TestCodeParser.test_python_callback_references_not_treated_as_dead	tests/test_parser.py:852	(test)

Test gaps

• code_review_graph/parser.py::CodeParser._ref_from_arguments (code_review_graph/parser.py:12184)
• tests/fixtures/sample_callback_refs.py::keyword_callback (tests/fixtures/sample_callback_refs.py:22)
• tests/fixtures/sample_callback_refs.py::register_keyword_callback (tests/fixtures/sample_callback_refs.py:42)
• tests/test_parser.py::TestCodeParser (tests/test_parser.py:13)

Token savings: this graph-backed report used ~187,072 fewer tokens (~99%) than reading every changed file in full (estimated, chars/4 approximation).

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[tirth8205]

Verified the new tests fail without the parser change and that builtins and keyword names get no spurious edges. Merging. Bound method callbacks like cb=self.on_event are still not covered, consistent with the scope cut in #424.