test(cpg): read /proc state without a check-then-read race

`test_hanging_process_group_killed_inside_shared_allowance` failed about one
run in thirty under load with FileNotFoundError: it checked that
/proc/<pid>/stat existed, the killed process was reaped, and the read then
failed. A reaped process is the outcome the test wants, so it now reads and
treats a missing entry as gone. The engine-stage test had the same race.
150 of 150 runs pass under engine load.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01EmM1j46qgKMh3zqD1QMwEo

tests/test_cpg_budget.py

@@ -67,7 +67,12 @@ while True: time.sleep(1)
         for pid in map(int, pids.read_text().split()):
             status = Path(f"/proc/{pid}/stat")
             for _ in range(30):
-                if not status.exists() or status.read_text().split()[2] == "Z":
+                # Read, never check-then-read: a killed process can be reaped between the two, and its
+                # disappearing is exactly the outcome this test wants, not an error.
+                try:
+                    if status.read_text().split()[2] == "Z":
+                        break
+                except FileNotFoundError:
                     break
                 time.sleep(0.005)
             else:
@@ -147,7 +152,10 @@ print({END!r})
         for pid in map(int, pid_log.read_text().split()):
             status = Path(f"/proc/{pid}/stat")
             for _ in range(30):
-                if not status.exists() or status.read_text().split()[2] == "Z":
+                try:  # read, never check-then-read: a reaped process is the outcome wanted
+                    if status.read_text().split()[2] == "Z":
+                        break
+                except FileNotFoundError:
                     break
                 time.sleep(0.005)
             else: