[chassisd] Capture DPU reboot cause via boot_id and add midplane-down reason for Smart Switch

[chartsai-nvidia]

Why I did it

On Smart Switch platforms, chassisd needs a reliable way to detect when a DPU has
rebooted and to record the correct reboot cause for it. The previous approach inferred
reboots from midplane operational-status transitions and time-window heuristics, which was
fragile and could miss or misattribute reboots. This change switches to a deterministic
signal: the DPU publishes its kernel boot_id, and the NPU captures the reboot cause
whenever that boot_id changes. It also adds a proper midplane-down reason
(Planned vs. Unplanned) per HLD rev 0.7.

Work item tracking

• Microsoft ADO (number only): N/A

How I did it

• DPU side (DpuStateUpdater / DpuStateManagerTask): read the kernel boot_id from
  /proc/sys/kernel/random/boot_id and publish it into the DPU_STATE table in
  CHASSIS_STATE_DB, updating it only when it changes.
• NPU side (SmartSwitchModuleUpdater): add check_dpu_boot_id_changes() to capture and
  persist the reboot cause when a DPU's boot_id changes (only while the midplane is
  reachable), and load persisted boot_ids on startup from previous-reboot-cause.json.
• Add RebootCauseSubscriberTask, which subscribes to DPU_STATE changes and triggers
  reboot-cause capture on boot_id updates, replacing the old status-transition /
  time-window reboot heuristic.
• Persist boot_id alongside the reboot-cause record and simplify the history file naming
  by removing the separate prev_reboot_time.txt bookkeeping.
• Add _resolve_midplane_down_reason() to classify midplane up->down transitions as
  Planned (from the STATE_DB transition flag/type) or Unplanned (from the platform's
  get_midplane_down_reason()), and write it into dpu_midplane_link_reason.

How to verify it

• Run the chassisd unit tests:
• cd sonic-chassisd && pytest tests/test_chassisd.py
• Tests added/updated in tests/test_chassisd.py, with new mock support in
  tests/mock_module_base.py (midplane-down reason constants) and tests/mock_platform.py
  (get_module_state_transition / get_midplane_down_reason) to cover boot_id capture
  and the Planned/Unplanned midplane-down reason logic.

[mssonicbld]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[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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