test(python): lock in cross-module type-reference binding (#4262)

#4262 reported that a type defined exactly once never bound across modules,
leaving hundreds of per-file stubs. It no longer reproduces on v8: the
reference binds through the re-export chain (and, absent any import, through the
unique-stub rewire). Add regression coverage matching the reporter's node
layout (config/settings/_models.py -> config_settings_models_settings) plus an
import-free inheritance control, so the resolved behavior cannot silently
regress. No production change required.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_python_reexport_type_binding.py

@@ -0,0 +1,81 @@
+"""Regression lock-in for #4262: a cross-module reference to a type defined
+exactly once must bind to that single located definition, not dangle on a
+per-file stub.
+
+The reporter saw a `Settings` class defined once at `config/settings/_models.py`
+(node `config_settings_models_settings`) with hundreds of per-file `Settings`
+stub nodes and zero links to the real class. On current `v8` the reference binds
+through the re-export chain (and, failing that, through the unique-stub rewire),
+so these tests pin that behavior against regressions.
+"""
+from __future__ import annotations
+
+from pathlib import Path
+
+from graphify.build import build_from_json
+from graphify.extract import extract
+
+
+def _edge_set(G):
+    return {
+        (d.get("relation"), d.get("_src", u), d.get("_tgt", v))
+        for u, v, d in G.edges(data=True)
+    }
+
+
+def _settings_stub_count(res: dict) -> int:
+    return sum(
+        1 for n in res["nodes"]
+        if n.get("label") == "Settings" and not n.get("source_file")
+    )
+
+
+def test_reexported_type_reference_binds_to_single_located_def(tmp_path: Path) -> None:
+    (tmp_path / "config" / "settings").mkdir(parents=True)
+    (tmp_path / "app").mkdir()
+    (tmp_path / "config" / "__init__.py").write_text("", encoding="utf-8")
+    (tmp_path / "config" / "settings" / "__init__.py").write_text(
+        "from ._models import Settings\n", encoding="utf-8"
+    )
+    (tmp_path / "config" / "settings" / "_models.py").write_text(
+        "class Settings:\n    def load(self):\n        return 1\n", encoding="utf-8"
+    )
+    (tmp_path / "app" / "service.py").write_text(
+        "from config.settings import Settings\n\n\n"
+        "def run(s: Settings):\n    return s.load()\n",
+        encoding="utf-8",
+    )
+
+    paths = [
+        tmp_path / "config" / "__init__.py",
+        tmp_path / "config" / "settings" / "__init__.py",
+        tmp_path / "config" / "settings" / "_models.py",
+        tmp_path / "app" / "service.py",
+    ]
+    res = extract(paths, root=tmp_path, parallel=False, cache_root=tmp_path / "graphify-cache")
+
+    assert _settings_stub_count(res) == 0, "no sourceless per-file Settings stub should survive"
+    G = build_from_json(res, root=str(tmp_path), directed=True)
+    edges = _edge_set(G)
+    assert ("references", "app_service_run", "config_settings_models_settings") in edges, (
+        f"cross-module Settings reference did not bind to the single located def: {edges}"
+    )
+
+
+def test_unresolved_type_references_coalesce_to_unique_def(tmp_path: Path) -> None:
+    """No imports at all: several files inherit a `Settings` defined exactly once.
+    The unique-stub rewire must bind every reference to the one real class rather
+    than leaving a per-file stub in each file."""
+    (tmp_path / "pkg").mkdir()
+    (tmp_path / "pkg" / "_models.py").write_text("class Settings:\n    pass\n", encoding="utf-8")
+    (tmp_path / "a.py").write_text("class A(Settings):\n    pass\n", encoding="utf-8")
+    (tmp_path / "b.py").write_text("class B(Settings):\n    pass\n", encoding="utf-8")
+
+    paths = [tmp_path / "pkg" / "_models.py", tmp_path / "a.py", tmp_path / "b.py"]
+    res = extract(paths, root=tmp_path, parallel=False, cache_root=tmp_path / "graphify-cache")
+
+    assert _settings_stub_count(res) == 0
+    G = build_from_json(res, root=str(tmp_path), directed=True)
+    edges = _edge_set(G)
+    assert ("inherits", "a_a", "pkg_models_settings") in edges
+    assert ("inherits", "b_b", "pkg_models_settings") in edges