[chassis] Record operator for user-initiated DPU reboot/shutdown

[vvolam]

What I did

Capture the operator who triggers a user-initiated DPU down event so the
DPU reboot-cause history (written by chassisd) can attribute it, mirroring
the NPU reboot-cause user field. Covers both DPU down paths:

• Reboot — reboot_smartswitch_helper
• Shutdown — config chassis modules shutdown <DPU>

Companion consumer PR in sonic-platform-daemons (chassisd reads the marker and
populates the user field): sonic-net/sonic-platform-daemons#841

Addresses sonic-net/sonic-buildimage#28149.

How I did it

Both producers write the operator to
/host/reboot-cause/module/<dpu>/prev_reboot_user.txt before the DPU goes
down; chassisd reads and clears this one-shot marker.

• scripts/reboot_smartswitch_helper: write REBOOT_USER (exported by the
  sourcing reboot script via logname, falling back to whoami) in
  reboot_dpu(), so both the single-DPU (-d) and all-DPU SmartSwitch paths
  are covered.
• config/chassis_modules.py: persist_dpu_reboot_user() writes the operator
  (SUDO_USER, falling back to getpass.getuser()) and is called in
  shutdown_chassis_module() for the SmartSwitch DPU branch. It is
  best-effort — a marker-write failure logs a warning and never blocks the
  shutdown. Hardware/auto reboots write no marker, so chassisd reports N/A.

How to verify it

Unit tests:

• tests/chassis_modules_test.py::test_persist_dpu_reboot_user — writes the
  marker with SUDO_USER.
• test_shutdown_smartswitch_module — asserts persist_dpu_reboot_user("DPU0")
  is invoked on shutdown.

Manual (SmartSwitch DUT):

1. config chassis modules shutdown DPU0 (or reboot -d DPU0).
2. cat /host/reboot-cause/module/dpu0/prev_reboot_user.txt → operator name.
3. After chassisd persists the reboot cause:
   redis-cli -h redis_chassis.server -p 6380 -n 13 HGETALL "REBOOT_CAUSE|DPU0|<name>"
   and show reboot-cause history DPU0 → User column shows the operator.

Previous command output (if the output of a command-line utility has changed)

No CLI output change. config chassis modules shutdown DPU0 still prints
Shutting down chassis module DPU0. The captured operator surfaces only in
show reboot-cause history <DPU> once chassisd persists the record.

New command output (if the output of a command-line utility has changed)

N/A — no command output changed; only the recorded reboot-cause user field.

[mssonicbld]

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[vvolam]

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[vvolam]

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