fix(tests): retry per-file pytest subprocess once on exit-4 when the file exists

The parallel test runner sharded a present, tracked test file
(tests/plugins/platforms/photon/test_inbound.py) onto a slice that then
reported 'file or directory not found' (pytest exit 4) at exec time —
even though the planner had just enumerated the file via --collect-only
('5269 passed, 0 failed' in the same run). On loaded shared CI runners
the per-file subprocess can fail to stat a file the planner already saw;
the deterministic LPT slicer then reproduces it on every rerun because
the same file set lands on the same shard.

Fix: when a per-file run exits 4 AND the file still exists on disk, retry
the subprocess once before surfacing it as a hard failure. This kills the
shard-flake class for everyone, not just this PR.

Does NOT widen the exit-5-is-pass rule — exit 4 on a genuinely missing
file still fails (verified). Retry reuses the same pgroup-kill cleanup as
the primary run so no grandchildren orphan.

Validation: photon dir runs green through scripts/run_tests_parallel.py;
unit-level negative case confirms a nonexistent file still returns rc=4.

scripts/run_tests_parallel.py

@@ -335,6 +335,50 @@ def _run_one_file(
         # dead processes are a no-op.
         _kill_tree(proc, pgid=pgid)
 
+    if rc == 4 and Path(file).exists():
+        # pytest exit 4 = "file or directory not found" at exec time, yet the
+        # file is present on disk now. On loaded shared CI runners we have seen
+        # the planner enumerate a file (its tests counted via --collect-only)
+        # but the per-file subprocess fail to stat it moments later — a
+        # transient the deterministic LPT slicer otherwise reproduces on every
+        # rerun (same file set → same shard). Retry the file ONCE before
+        # surfacing it as a hard failure. We do NOT widen the exit-5 rule:
+        # exit 4 on a file that genuinely does not exist must still fail.
+        retry_proc = subprocess.Popen(
+            cmd,
+            cwd=repo_root,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.STDOUT,
+            text=True,
+            start_new_session=True,
+        )
+        retry_pgid: int | None = None
+        if sys.platform != "win32":
+            try:
+                retry_pgid = os.getpgid(retry_proc.pid)
+            except (ProcessLookupError, PermissionError):
+                retry_pgid = None
+        try:
+            retry_output, _ = retry_proc.communicate(timeout=file_timeout)
+            retry_rc = retry_proc.returncode
+        except subprocess.TimeoutExpired:
+            _kill_tree(retry_proc, pgid=retry_pgid)
+            try:
+                retry_output, _ = retry_proc.communicate(timeout=10)
+            except subprocess.TimeoutExpired:
+                retry_output = "(file timeout exceeded on retry; output unavailable)"
+            retry_rc = 124
+            retry_output = (
+                f"(per-file timeout on exit-4 retry: {file_timeout:.0f}s exceeded; "
+                f"process tree SIGKILL'd)\n{retry_output}"
+            )
+        except BaseException:
+            _kill_tree(retry_proc, pgid=retry_pgid)
+            raise
+        else:
+            _kill_tree(retry_proc, pgid=retry_pgid)
+        rc, output = retry_rc, retry_output
+
     if rc == 5:
         # No tests collected — every test in the file was filtered out.
         # Treat as a pass; surface info in a slightly distinct status