[Nexthop][fboss2-dev] Add fboss2-dev config mac aging-time CLI command

[vybhav-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

Adds fboss2-dev config mac aging-time <seconds> to set the L2 MAC
address aging timer (l2AgeTimerSeconds in SwitchSettings, default
300s).

The change is applied HITLESSLY by the SAI layer
(SaiSwitch.cpp::setMacAgingSeconds(), no ChangeProhibited guard).

New files:

• commands/config/mac/CmdConfigMac.{h,cpp} — parent branch node
• commands/config/mac/aging_time/CmdConfigMacAgingTime.{h,cpp} — leaf handler with positive-integer validation
• test/config/CmdConfigMacAgingTimeTest.cpp — unit tests (arg validation, set, already-set)
• test/integration_test/ConfigMacTest.cpp — E2E set/restore test

Modified: CmdListConfig.cpp, cmake/CliFboss2.cmake, cmake/CliFboss2TestConfig.cmake, cmake/CliFboss2TestIntegrationTest.cmake, fboss2/BUCK, test/config/BUCK, test/integration_test/BUCK

Test Plan

Unit tests — 3 test cases covering arg validation, set, and already-set no-op:

$ bazel test //fboss/cli/fboss2/test/config:cmd_config_test \
    --test_filter='CmdConfigMacAgingTimeTestFixture.*' --test_output=all
...
[  PASSED  ] 3 tests.

Integration test — ConfigMacTest.SetAndRestoreAgingTime on a DUT (set aging-time, verify, restore).

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