[Nexthop][fboss2-dev] Add delete switch admin-distance and icmpv4-unavailable-src-addr

[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

Background: what admin distance is and what deleting an entry means

When two routing clients (BGP, OpenR, static config, ...) offer routes to the same prefix, admin distance decides who wins: the route with the lowest distance is programmed. sw.clientIdToAdminDistance maps a client-id to its distance, and the agent consults it through getAdminDistanceForClientId() each time it programs a route for that client.

Deleting a client's entry does not resurrect a per-client default: the thrift default map applies only when the whole field is absent from the config. A client with no entry gets AdminDistance::MAX_ADMIN_DISTANCE (255, least preferred), and the agent logs an error per lookup. Client-ids 1 through 4 (STATIC_ROUTE, INTERFACE_ROUTE, LINKLOCAL_ROUTE, REMOTE_INTERFACE_ROUTE) never consult the map at all; their distances are compile-time constants in the agent.

What: Two delete commands under a new delete switch branch, mirroring config switch:

fboss2-dev delete switch admin-distance <client-id>
fboss2-dev delete switch icmpv4-unavailable-src-addr

• admin-distance removes the client's entry from sw.clientIdToAdminDistance. Saved at coldboot level, for the same reason as the config counterpart: the map is consulted only at route-program time, and existing routes are not re-stamped when it changes. Client-ids 1 through 4 are refused, mirroring the refusal in config switch admin-distance, since removing an entry the agent never reads would only pretend to do something.
• icmpv4-unavailable-src-addr clears the optional sw.icmpV4UnavailableSrcAddress field, returning it to its unset default. Hitless.

Why: config switch admin-distance and config switch icmpv4-unavailable-src-addr could set these values but nothing could remove them.

How: Both leaves follow the single-positional-arg pattern of the existing delete commands. The client-id parsing and forbidden-client check are shared with the config command via parseAdminDistanceClientId() next to the existing forbiddenAdminDistanceClients(), so the config and delete commands cannot drift apart. The success message states the consequence: routes from the client will use MAX_ADMIN_DISTANCE (255) until a new entry is set, and a coldboot applies the change to existing routes.

Sample usage

$ fboss2-dev delete switch admin-distance 786
Successfully removed admin distance entry for client-id 786. Routes from this client will use MAX_ADMIN_DISTANCE (255). A coldboot is required to apply the change to existing routes.
$ fboss2-dev delete switch admin-distance 1
FBOSS does not allow removing the admin distance entry for client-id 1: STATIC_ROUTE is hardcoded to AdminDistance::STATIC_ROUTE in the agent
$ fboss2-dev delete switch icmpv4-unavailable-src-addr
Successfully removed ICMPv4 unavailable source address 192.0.2.1; the agent will use the RFC 7600 default (192.0.0.8)

Test Plan

Validation ran in our internal tree, which carries this same change. The upstream tree itself was not built locally; this PR's CI is the build verification.

Unit (cmd_config_test): arg validation (arity, non-integer, negative), forbidden-client refusal (ids 1 through 4 refused, entry kept), delete existing entry, delete missing entry throws, plus the icmpv4 set/clear paths:

[==========] 12 tests from 2 test suites ran. (1033 ms total)
[  PASSED  ] 12 tests.

Integration (ConfigAdminDistanceTest, run on a DUT): set an entry for an unused client-id, delete it, verify the running config returns exactly to baseline; forbidden client-ids rejected symmetrically by config and delete.

Review Findings

Pre-publication review (5 reviewers + verifier) raised one finding, fixed before push: the delete argument type duplicated the config command's client-id parse/validate block, now extracted into the shared parseAdminDistanceClientId().