[chassisd] Record operator in DPU reboot-cause 'user' field

[vvolam]

Description

DPU reboot-cause history records written by chassisd hardcoded the dict in
persist_dpu_reboot_cause() without a user key, so when
update_dpu_reboot_cause_to_db() deletes and rewrites the
REBOOT_CAUSE|DPU<N>|* keys in CHASSIS_STATE_DB, the user field
disappeared — diverging from process-reboot-cause (sonic-host-services),
which always writes user.

This change:

• Adds the user field to the persisted DPU reboot-cause record.
• Populates it with the operator who triggered a user-initiated DPU
  reboot/shutdown, read from a one-shot marker
  /host/reboot-cause/module/<dpu>/prev_reboot_user.txt written by the
  sonic-utilities producers (reboot_smartswitch_helper and
  config chassis modules shutdown — see companion PR). chassisd reads the
  marker via retrieve_dpu_reboot_user() and removes it after use.
• Defaults user to N/A for hardware/auto-detected reboots where no
  marker exists, matching the NPU determine-reboot-cause default and keeping
  the JSON schema consistent with process-reboot-cause.

Motivation and Context

Addresses sonic-net/sonic-buildimage#28149 — on SmartSwitch, DPU reboot-cause
entries were schema-inconsistent because chassisd omitted the user field
when rewriting CHASSIS_STATE_DB. Beyond just restoring schema parity, this
attributes user-initiated DPU reboots/shutdowns, mirroring the NPU
reboot-cause user field shown by show reboot-cause history.

Producer (marker-writer) companion PR in sonic-utilities: sonic-net/sonic-utilities#4653

How Has This Been Tested?

Unit tests — full sonic-chassisd suite passes (53 tests). New/updated cases:

• test_persist_dpu_reboot_cause_includes_user_field — user present, defaults to N/A.
• test_persist_dpu_reboot_cause_records_user_for_user_initiated_reboot — records the marker user (admin).
• test_retrieve_dpu_reboot_user — returns the user when the marker is present, N/A when empty/missing.

Manual (SmartSwitch DUT):

1. Trigger a user-initiated DPU reboot (reboot -d DPU0) or shutdown (config chassis modules shutdown DPU0).
2. After chassisd processes it: redis-cli -h redis_chassis.server -p 6380 -n 13 HGETALL "REBOOT_CAUSE|DPU0|<name>" → user = operator.
3. Hardware/auto reboot → user = N/A.

Additional Information (Optional)

This is the consumer side. The marker files are produced by the companion
sonic-utilities PR; both are required for end-to-end user attribution.

[mssonicbld]

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