[Mellanox] Implement get_midplane_down_reason for DPU module

[chartsai-nvidia]

Why I did it

Mellanox DpuModule did not implement the pmon ModuleBase.get_midplane_down_reason()
interface, so it fell back to the generic base behavior instead of reporting the actual DPU
hardware reset cause. As a result, when a DPU's midplane went down there was no way for pmon to
surface whether it was caused by a power loss, a thermal shutdown, or a reset from the main board.

Work item tracking

• Microsoft ADO (number only): N/A

How I did it

• Implemented DpuModule.get_midplane_down_reason() in
  platform/mellanox/mlnx-platform-api/sonic_platform/module.py.
• The implementation scans the existing reboot_cause_map (the per-DPU
  /var/run/hw-management/<dpu>/system/reset_* sysfs files) and returns the
  (ChassisBase.REBOOT_CAUSE_*, description) tuple of the first file that reads 1.
• Falls back to (ChassisBase.REBOOT_CAUSE_NON_HARDWARE, '') when no reset-cause file is
  asserted, matching the existing get_reboot_cause() contract.
• Logs a notice with the resolved reason so the cause is recoverable from syslog.
• Reuses the existing ChassisBase.REBOOT_CAUSE_* constants rather than introducing new
  midplane-specific constants.

How to verify it

Run the Mellanox platform API unit tests:

cd platform/mellanox/mlnx-platform-api
python3 -m pytest tests/test_module.py::TestModule::test_dpu_module -v

Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202012
• 202106
• 202111
• 202205
• 202211
• 202305
• 202311
• 202405
• 202411

Description for the changelog

Implement get_midplane_down_reason for Mellanox DPU module

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This PR has backport request label(s) for branch(es): 202605, but is missing required test information. Please make sure you tick the tested branch(es) in the Tested branch section and provide test evidence (e.g., 202605: <test result>) in the Test result section as well in your PR description.

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