[ModuleBase] Add get_midplane_down_reason() API and MIDPLANE_DOWN_REASON_* constants

[chartsai-nvidia]

Why I did it

Modular chassis / SmartSwitch platforms need a standard way to report why a module's midplane
went down. ModuleBase exposes reachability but no reason. Part of the DPU robustness HLD.

Work item tracking

• Microsoft ADO (number only): N/A

How I did it

• Added MIDPLANE_DOWN_REASON_* constants to ModuleBase (aligned with chassis reboot causes).
• Added get_midplane_down_reason() returning a (reason, description) tuple, default
  raise NotImplementedError.
• Covered the new method in test_not_implemented_methods_raise.

How to verify it

cd src/sonic-platform-common && pytest tests/module_base_test.py

[mssonicbld]

/azp run

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[mssonicbld]

Hi, there are workflow run(s) waiting for approval, you may be first-time contributor. I will notify maintainers to help approve once PR is approved. Thanks!

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[mssonicbld]

Hi @sonic-net/sonic-platform-common-maintainer, this approved PR has workflow run(s) waiting for approval. Please help review. Thanks!

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