[Nexthop][fboss2-dev] Add show environment fan CLI command

[daria-nexthop]

Pre-submission checklist

• I've ran the linters locally and fixed lint errors related to the files I modified in this PR. You can install the linters by running pip install -r requirements-dev.txt && pre-commit install
• pre-commit run

Summary

Add fboss2 show environment fan CLI command.

Example output:

Fan Name     Status  Target PWM (%)  RPM    Last Accessed       
-----------------------------------------------------------------------
fan_1_inner  OK      60.0            75399  2026-06-09 09:50:11 
fan_1_outer  OK      60.0            65691  2026-06-09 09:50:11 
fan_2_inner  OK      60.0            75399  2026-06-09 09:50:11 
fan_2_outer  OK      60.0            65691  2026-06-09 09:50:11 

Columns:

• Fan Name — fan identifier as reported by the fan service
• Status — OK or FAILED (colored green/red respectively)
• Target PWM (%) — the last PWM value computed and sent to the hardware based on current sensor and optics readings
• RPM — last observed fan speed; shown as N/A if the fan could not be read (e.g. fan failed)
• Last Accessed — local-time timestamp of the last successful fan access, formatted as YYYY-MM-DD HH:MM:SS

Test Plan

• Added CmdShowEnvironmentFanTest.cpp with queryClient and printOutput coverage
• Verified on a real device

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[benoit-nexthop]

Daria is on the wait list for CLA.

[meta-cla]

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