fix(parsers/python): extract block-scoped def/class units

[gadievron]

What

Extract def/class units declared inside block statements (if/elif/else, try/except/finally, for/while, with, match/case), which the Python extractor dropped.

Root cause

process_file (function_extractor.py:768), _process_function_tree (:383), and _process_class_tree (:406) recurse only into FunctionDef/ClassDef bodies — never into block statements. The call-graph builder consumes the same functions dict (call_graph_builder.py:88) with no independent traversal, so a block-nested def is missing from BOTH the inventory and the call graph. Sibling effect: extract_module_level_code computed covered lines from top-level def/class spans only, so a block def's body — including any sink it calls — leaked verbatim into the synthetic :__module__ unit and produced spurious __module__ -> callee edges. A def under if sys.version_info >= ..., a try/except ImportError fallback, a with-guarded handler, or a class-based-view if/else dispatcher was therefore never analyzed as a unit.

Reproduction

$ printf 'if X:\n    def handler(req):\n        return __import__("os").system(req)\n' > /tmp/r/m.py
$ python -c "from parsers.python.function_extractor import FunctionExtractor as FE; ..."
# handler absent from functions; its os.system text inside the :__module__ unit

How

• New _descend_into_blocks: walks block-container bodies (including except/finally arms and match cases) at all depths and hands any def/class to the existing tree helpers. It descends ONLY into block nodes — direct FunctionDef/ClassDef children are emitted by the caller, so the two node sets are disjoint (no double-processing). Wired into all three traversal entry points.
• This matches Python's existing baseline (function-nested defs are already units) and reuses the existing keep-both (#L<line>) disambiguation, so sibling-block same-name defs (a CBV if/else view) and a block def colliding with a top-level def both survive.
• Closes the :__module__ leak by covering every def/class span via ast.walk (not only top-level children) in extract_module_level_code.
• Adds the canonical per-parser test-infra files that were absent for Python: test_callgraph_symmetry.py (call_graph keys are a subset of functions keys) and test_python_schema_completeness.py (every unit carries the schema-contract fields).

Scope: descends into block statements at all depths (consistent with Python's keep-function-nested design). Lambda/comprehension scopes are not def/class nodes and are out of scope.

Regression test

tests/parsers/python/test_block_scoped_defs.py — every block shape; async + decorated; class-in-block + its methods; function-internal block; sibling-block keep-both; block-vs-top-level keep-both; and the :__module__ no-leak check.

RED (before fix):

15 failed

GREEN (after fix):

$ pytest tests/parsers/python/test_block_scoped_defs.py -q
15 passed
$ pytest tests/parsers/python/ -q
64 passed

Full repo suite: 636 passed, 22 skipped. Flask real-repo: both src/flask/views.py view dispatchers surfaced (view + view#L115, keep-both); all 6 src/flask/globals.py if TYPE_CHECKING: classes surfaced.

Compatibility

No id change for code without block-scoped defs (the block-descent only adds units the old walk never reached; direct-child traversal is unchanged). The :__module__ content shrinks for files with block defs (their bodies move into their own units) — a correction that removes today's leak.