[Smartswitch] Use MODULE_ADMIN_DOWN when DPU operational state is offline

[gpunathilell]

Description

• Remove MODULE_PRE_SHUTDOWN constant and all related handling from chassisd.
• When DPU admin state is MODULE_STATUS_EMPTY, always submit MODULE_ADMIN_DOWN to the callback (no longer use MODULE_PRE_SHUTDOWN when operational state is offline).
• Simplify submit_dpu_callback: it no longer invokes module_pre_shutdown; it only calls set_admin_state_gracefully when admin state is MODULE_ADMIN_DOWN.
• Update unit tests to match: expect MODULE_ADMIN_DOWN for the empty-offline case, and remove tests for MODULE_PRE_SHUTDOWN.

Motivation and Context

The DPU can be in an intermediate state where operational_state is offline but we still need to force power off the DPU. Previously, when admin state was EMPTY and operational state was OFFLINE, the daemon used MODULE_PRE_SHUTDOWN, which did not reliably power off the DPU in this case. By always using MODULE_ADMIN_DOWN when admin state is EMPTY (regardless of operational state), we ensure the DPU is forced power off when required.

How Has This Been Tested?

• Updated and ran unit tests in sonic-chassisd/tests/test_chassisd.py:
  • test_set_initial_dpu_admin_state_empty_offline: now expects MODULE_ADMIN_DOWN instead of MODULE_PRE_SHUTDOWN.
  • test_set_initial_dpu_admin_state_empty_not_offline: assertion/comment aligned with new behavior.
  • test_submit_dpu_callback: removed MODULE_PRE_SHUTDOWN scenario; retained MODULE_ADMIN_DOWN test.

Additional Information (Optional)

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[vvolam]

LGTM