test(649): make the POST-wiring assertion unable to opt out or accept the wrong host

Re-review found the verifier could disable itself two ways, both of which look like
coverage:

- it was guarded by `if url_file.exists()`, so deleting the recorder in the stub turned
  the whole assertion into a no-op and every test stayed green;
- it compared only the URL SUFFIX, so a POST to the right path on the wrong HOST or the
  wrong REPO passed — which is exactly the class the assertion was added to catch.

It now requires the URL to have been recorded whenever a status was posted, and compares
the full URL against the env the job was given. Mutation-verified three ways: wrong host,
wrong repo, and deleting the recorder each redden the suite.

Refs #649

scripts/tests/test_pr_changed_files.py

@@ -607,15 +607,24 @@ def _run_classify(tmp_path, enum_stub: str | None, author: str = "timothy"):
     script.write_text(_classify_step()["run"])
     r = subprocess.run(["bash", str(script)], cwd=tmp_path, env=env,
                        capture_output=True, text=True)
+    # Assert the WIRING, not only the classification. The stub accepts every POST, so a status aimed
+    # at the wrong endpoint, sha, host or repo would otherwise leave these tests green while the real
+    # required check was never written.
+    #
+    # Two ways this check could disable itself, both found by cold review of an earlier draft:
+    #   * it was guarded by `if url_file.exists()`, so deleting the recorder in the stub turned it
+    #     into a no-op and every test stayed green — a verifier that silently opts out;
+    #   * it compared only the URL SUFFIX, so a POST to the right path on the WRONG HOST OR REPO
+    #     passed. Compare the whole URL against the env this job was given.
     posted = tmp_path / "posted.json"
     url_file = tmp_path / "posted_url.txt"
-    if url_file.exists():
-        # Assert the WIRING, not only the classification. Cold review's point: the stub accepts every
-        # POST, so a status aimed at the wrong endpoint or the wrong sha would leave these tests green
-        # while the real required check was never written.
-        url = url_file.read_text().strip()
-        assert url.endswith(f"/statuses/{SHA}"), (
-            f"the status was POSTed to {url!r}, not to /statuses/<full head sha>")
+    if posted.exists():
+        assert url_file.exists(), (
+            "a status was POSTed but its URL was not recorded — the wiring assertion below would "
+            "have silently skipped")
+        expected = f"{env['BASE_URL']}/repos/{env['REPO']}/statuses/{SHA}"
+        assert url_file.read_text().strip() == expected, (
+            f"the status was POSTed to {url_file.read_text().strip()!r}, expected {expected!r}")
     return (json.loads(posted.read_text()) if posted.exists() else None), r