fix(gateway): close --replace race completely by claiming PID before adapter startup

Follow-up on top of opriz's atomic PID file fix. The prior change caught
the race AFTER runner.start(), so the loser still opened Telegram polling
and Discord gateway sockets before detecting the conflict and exiting.

Hoist the PID-claim block to BEFORE runner.start(). Now the loser of the
O_CREAT|O_EXCL race returns from start_gateway() without ever bringing up
any platform adapter — no Telegram conflict, no Discord duplicate session.

Also add regression tests:
- test_write_pid_file_is_atomic_against_concurrent_writers: second
  write_pid_file() raises FileExistsError rather than clobbering.
- Two existing replace-path tests updated to stateful mocks since the
  real post-kill state (get_running_pid None after remove_pid_file)
  is now exercised by the hoisted re-check.

gateway/run.py

@@ -10951,6 +10951,30 @@ async def start_gateway(config: Optional[GatewayConfig] = None, replace: bool =
     else:
         logger.info("Skipping signal handlers (not running in main thread).")
     
+    # Claim the PID file BEFORE bringing up any platform adapters.
+    # This closes the --replace race window: two concurrent `gateway run
+    # --replace` invocations both pass the termination-wait above, but
+    # only the winner of the O_CREAT|O_EXCL race below will ever open
+    # Telegram polling, Discord gateway sockets, etc. The loser exits
+    # cleanly before touching any external service.
+    import atexit
+    from gateway.status import write_pid_file, remove_pid_file, get_running_pid
+    _current_pid = get_running_pid()
+    if _current_pid is not None and _current_pid != os.getpid():
+        logger.error(
+            "Another gateway instance (PID %d) started during our startup. "
+            "Exiting to avoid double-running.", _current_pid
+        )
+        return False
+    try:
+        write_pid_file()
+    except FileExistsError:
+        logger.error(
+            "PID file race lost to another gateway instance. Exiting."
+        )
+        return False
+    atexit.register(remove_pid_file)
+
     # Start the gateway
     success = await runner.start()
     if not success:
@@ -10960,29 +10984,6 @@ async def start_gateway(config: Optional[GatewayConfig] = None, replace: bool =
             logger.error("Gateway exiting cleanly: %s", runner.exit_reason)
         return True
     
-    # Write PID file so CLI can detect gateway is running
-    import atexit
-    from gateway.status import write_pid_file, remove_pid_file, get_running_pid
-    # Defensive re-check: another --replace racer may have started
-    # while we were initializing. If so, yield and exit.
-    _current_pid = get_running_pid()
-    if _current_pid is not None and _current_pid != os.getpid():
-        logger.error(
-            "Another gateway instance (PID %d) started during our startup. "
-            "Exiting to avoid double-running.", _current_pid
-        )
-        await runner.stop()
-        return False
-    try:
-        write_pid_file()
-    except FileExistsError:
-        logger.error(
-            "PID file race lost to another gateway instance. Exiting."
-        )
-        await runner.stop()
-        return False
-    atexit.register(remove_pid_file)
-    
     # Start background cron ticker so scheduled jobs fire automatically.
     # Pass the event loop so cron delivery can use live adapters (E2EE support).
     cron_stop = threading.Event()