[Nexthop][fboss2-dev] Add config admin-distance 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 a new write command:

fboss2-dev config admin-distance <client-id> <distance>

Sets the administrative distance for a routing client in the agent running config via a HITLESS config session.

What

• New command CmdConfigAdminDistance with AdminDistanceArg validating two positional args: client-id (non-negative integer) and distance (0-255)
• Read-modify-write on clientIdToAdminDistance map - all other client entries survive untouched
• No-op detection - if the value is already set, returns early without saving
• Registered in kConfigCommandTree() alongside existing config subcommands
• Wired into CMake (fboss2_config_lib, fboss2_cmd_config_test, fboss2_integration_test) and BUCK

Why HITLESS (not in ApplyThriftConfig.cpp)

clientIdToAdminDistance is absent from ApplyThriftConfig.cpp — it does not go through the SwitchState delta pipeline and is never programmed to hardware. Instead, SwSwitch::clientIdToAdminDistance() reads it directly from the live agent config at route-update time (via getAdminDistanceForClientId(getConfig(), clientId) in Utils.cpp). Saving the config is sufficient for the change to take effect on the next route update, so no agent restart is needed — HITLESS is correct.

Test Plan

Unit tests — //fboss/cli/fboss2/test/config:cmd_config_test

[----------] 4 tests from CmdConfigAdminDistanceTestFixture
[ RUN      ] CmdConfigAdminDistanceTestFixture.argValidation
[       OK ] CmdConfigAdminDistanceTestFixture.argValidation (85 ms)
[ RUN      ] CmdConfigAdminDistanceTestFixture.updateExistingClient
[       OK ] CmdConfigAdminDistanceTestFixture.updateExistingClient (95 ms)
[ RUN      ] CmdConfigAdminDistanceTestFixture.addNewClient
[       OK ] CmdConfigAdminDistanceTestFixture.addNewClient (76 ms)
[ RUN      ] CmdConfigAdminDistanceTestFixture.alreadySet
[       OK ] CmdConfigAdminDistanceTestFixture.alreadySet (60 ms)
[----------] 4 tests from CmdConfigAdminDistanceTestFixture (318 ms total)
[  PASSED  ] 260 tests.
//fboss/cli/fboss2/test/config:cmd_config_test    PASSED in 16.4s

Integration test — ConfigAdminDistanceTest.SetAndRestoreAdminDistance on NH-4010-F

[==========] Running 1 test from 1 test suite.
[ RUN      ] ConfigAdminDistanceTest.SetAndRestoreAdminDistance
[       OK ] ConfigAdminDistanceTest.SetAndRestoreAdminDistance (154 ms)
[  PASSED  ] 1 test.

I0601 10:01:04] [Step 1] Reading current admin distances...
I0601 10:01:04]   client-id 0 -> 20
I0601 10:01:04] [Step 2] Setting client-id 0 to 42...
I0601 10:01:04] Running CLI: config admin-distance 0 42
I0601 10:01:04] Running CLI: config session commit
I0601 10:01:04] [Step 3] Restoring client-id 0 to 20...
I0601 10:01:04] Running CLI: config admin-distance 0 20
I0601 10:01:04] Running CLI: config session commit
I0601 10:01:04] TEST PASSED