esim: hold filesystem lock during AtClient echo disable

When multiple processes create TiciLPA simultaneously, each AtClient
sends ATE0 to the serial port during init without holding any lock.
This can inject into the serial stream while another process is
mid-command, causing timeouts. Hold the filesystem lock during
_disable_echo to serialize init across processes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: greatgitsby

system/hardware/tici/lpa.py

@@ -33,6 +33,7 @@
 MM = "org.freedesktop.ModemManager1"
 MM_MODEM = MM + ".Modem"
 ES10X_MSS = 120
+LOCK_FILE = '/tmp/.lpa.lock'
 DEBUG = True
 
 # TLV Tags
@@ -128,7 +129,13 @@ def __init__(self, device: str, baud: int, timeout: float, debug: bool) -> None:
     self._use_dbus = False
     try:
       self._serial = serial.Serial(device, baudrate=baud, timeout=timeout)
-      self._disable_echo()
+      fd = os.open(LOCK_FILE, os.O_CREAT | os.O_RDWR)
+      try:
+        fcntl.flock(fd, fcntl.LOCK_EX)
+        self._disable_echo()
+      finally:
+        fcntl.flock(fd, fcntl.LOCK_UN)
+        os.close(fd)
       if self.debug:
         print("AtClient: using serial transport", file=sys.stderr)
     except (serial.SerialException, PermissionError, OSError) as e:
@@ -692,7 +699,6 @@ def set_profile_nickname(client: AtClient, iccid: str, nickname: str) -> None:
 
 
 MM_DEVICE_UID = '/sys/devices/platform/soc/a800000.ssusb/a800000.dwc3/xhci-hcd.0.auto/usb1/1-1'
-LOCK_FILE = '/tmp/.lpa.lock'
 
 
 class TiciLPA(LPABase):