[Mellanox] Query PSU fan speed only on presence (#21593)

- Why I did it
The PsuFan in Mellanox platform queries for fan speed even though the relevant sysfs files do not exist. In psud the speed is already overwritten on presence, since the current implementation generates an error log, it is changed to query speed only on PSU presence and return 0 if there is no presence (Previously returned default value)

- How I did it
Check presence using get_presence api and return fan speed only if it returns True

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

@@ -1,6 +1,7 @@
 #
-# Copyright (c) 2019-2023 NVIDIA CORPORATION & AFFILIATES.
-# Apache-2.0
+# SPDX-FileCopyrightText: NVIDIA CORPORATION & AFFILIATES
+# Copyright (c) 2019-2025 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -256,6 +257,11 @@ def set_speed(self, speed):
             logger.log_error('Failed to set PSU FAN speed - {}'.format(e))
             return False
 
+    def get_speed(self):
+        if not self.get_presence():
+            logger.log_notice(f"No PSU presence detected, returning default value for {self._name}")
+            return 0
+        return super().get_speed()
 
 class Fan(MlnxFan):
     """Platform-specific Fan class"""

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

@@ -1,6 +1,7 @@
 #
-# Copyright (c) 2020-2023 NVIDIA CORPORATION & AFFILIATES.
-# Apache-2.0
+# SPDX-FileCopyrightText: NVIDIA CORPORATION & AFFILIATES
+# Copyright (c) 2020-2025 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -14,6 +15,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #
+
 import os
 import pytest
 import subprocess
@@ -155,3 +157,8 @@ def mock_read_str_from_file(file_path, default='', raise_exception=False):
         subprocess.check_call = MagicMock()
         utils.read_str_from_file = MagicMock(side_effect=RuntimeError(''))
         assert not fan.set_speed(60)
+        fan.get_presence = MagicMock(return_value=False)
+        assert fan.get_speed() == 0
+        fan.get_presence = MagicMock(return_value=True)
+        utils.read_int_from_file = MagicMock(return_value=60)
+        assert fan.get_speed() == 100