test: deflake process-registry kill + PTY resize tests

Two CI flakes surfaced on PR #34572 (both in files this PR doesn't touch;
pre-existing host-dependent flakes):

1. test_process_registry::TestPopenLeakOnSetupFailure — the failure-cleanup
   tests use a fake proc.pid (8888/9999) and assert proc.kill() runs. But
   spawn_local's primary cleanup is os.killpg(os.getpgid(pid), SIGKILL),
   falling back to proc.kill() only on ProcessLookupError/PermissionError/
   OSError. When the fake PID happens to exist on a busy host, os.getpgid
   succeeds, os.killpg fires against an UNRELATED real process group, and
   proc.kill() is never reached -> flaky AssertionError (and a real risk of
   SIGKILLing an innocent process group from a unit test). Patch os.getpgid
   to raise ProcessLookupError so the fallback path runs deterministically
   and no real killpg is ever issued.

2. test_web_server::test_resize_escape_is_forwarded — the receive loop calls
   the blocking conn.receive_bytes() with no exception guard. Once the child
   prints its winsize and exits, the PTY closes; on a missed-marker run the
   next recv blocks until the 30s pytest-timeout instead of failing fast.
   Add a try/except break (matching the working sibling tests) and bump the
   child's pre-read sleep 0.15s -> 0.5s so the resize reliably lands first.

Verified: 4/4 pass across 3 consecutive runs; root cause for #1 reproduced
(os.getpgid(1) succeeds -> old code skips proc.kill).

tests/hermes_cli/test_web_server.py

@@ -2195,7 +2195,7 @@ class TestPtyWebSocket:
 
         winsize_script = (
             "import fcntl, struct, termios, time; "
-            "time.sleep(0.15); "
+            "time.sleep(0.5); "
             "rows, cols, *_ = struct.unpack('HHHH', "
             "fcntl.ioctl(0, termios.TIOCGWINSZ, b'\\0' * 8)); "
             "print(cols); print(rows)"
@@ -2217,7 +2217,14 @@ class TestPtyWebSocket:
 
             deadline = time.monotonic() + 5.0
             while time.monotonic() < deadline:
-                frame = conn.receive_bytes()
+                # receive_bytes() blocks; once the child prints its winsize and
+                # exits, the PTY closes and further reads raise. Without this
+                # guard a missed-marker run blocks until the 30s pytest-timeout
+                # (flaky failure) instead of failing fast on the assert below.
+                try:
+                    frame = conn.receive_bytes()
+                except Exception:
+                    break
                 if frame:
                     buf += frame
                 if b"99" in buf and b"41" in buf: