[Mellanox] wait until hw-management watchdog files ready (#17618)

- Why I did it
watchdog-control service always disarm watchdog during system startup stage. It could be the case that watchdog is not fully initialized while the watchdog-control service is accessing it. This PR adds a wait to make sure watchdog has been fully initialized.

- How I did it
adds a wait to make sure watchdog has been fully initialized.

- How to verify it
Manual test
sonic regression

platform/mellanox/mlnx-platform-api/sonic_platform/watchdog.py

@@ -284,6 +284,7 @@ def get_watchdog():
     Return WatchdogType1 or WatchdogType2 based on system
     """
     
+    utils.wait_until(lambda: os.path.exists('/run/hw-management/watchdog/main/state'), timeout=10, interval=1)
     watchdog_main_device_name = None
 
     for device in os.listdir("/dev/"):

platform/mellanox/mlnx-platform-api/tests/test_watchdog.py

@@ -36,6 +36,7 @@ from sonic_platform.watchdog import get_watchdog,    \
 
 
 class TestWatchdog:
+    @mock.patch('sonic_platform.utils.wait_until', mock.MagicMock())
     @mock.patch('sonic_platform.watchdog.is_mlnx_wd_main')
     @mock.patch('sonic_platform.watchdog.os.listdir')
     def test_get_watchdog_no_device(self, mock_listdir, mock_is_main):
@@ -50,6 +51,7 @@ class TestWatchdog:
         mock_is_main.return_value = False
         assert get_watchdog() is None
 
+    @mock.patch('sonic_platform.utils.wait_until', mock.MagicMock())
     @mock.patch('sonic_platform.watchdog.is_mlnx_wd_main')
     @mock.patch('sonic_platform.watchdog.is_wd_type2')
     @mock.patch('sonic_platform.watchdog.os.listdir', mock.MagicMock(return_value=['watchdog1', 'watchdog2']))