refactor: split python test enumeration into its own module

python_runner.py owns walking a Python test file's AST for test_*
functions and classes, separated out ahead of the strategies/unit_tests
mapping_validator restructure.

tests/rust-python-harness/strategies/unit_tests/python_runner.py

@@ -0,0 +1,22 @@
+from __future__ import annotations
+
+import ast
+from pathlib import Path
+
+
+def enumerate_python_tests(repo_root: Path, relative_path: str) -> frozenset[str]:
+    source = (repo_root / relative_path).read_text(encoding="utf-8")
+    tree = ast.parse(source, filename=relative_path)
+
+    module_level: list[str] = []
+    for node in ast.iter_child_nodes(tree):
+        if isinstance(node, (ast.FunctionDef, ast.AsyncFunctionDef)) and node.name.startswith("test_"):
+            module_level.append(node.name)
+        elif isinstance(node, ast.ClassDef):
+            for child in ast.iter_child_nodes(node):
+                if isinstance(child, (ast.FunctionDef, ast.AsyncFunctionDef)) and child.name.startswith(
+                    "test_"
+                ):
+                    module_level.append(f"{node.name}::{child.name}")
+
+    return frozenset(module_level)