[Nexthop][fboss2-dev] Add fboss2-dev config hostname CLI

[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 hostname <name>, which sets the optional
sw.hostname field in the agent switch config.

• Action level: HITLESS — applied via
  ThriftConfigApplier::updateSwitchSettings
  (newSwitchSettings->setHostname); no SAI ChangeProhibited guard.
• Upsert: if hostname is already set to the requested value, the
  command is a no-op.
• Validation: the hostname argument is validated against RFC 952/1123
  using RE2 (alphanumeric plus hyphens, max 63 chars per label, total max
  253 chars).
• Field: switch_config.thrift:SwitchConfig.hostname (optional string)
  — "Overrides the system hostname, useful in ICMP responses."

Test Plan

• Unit tests (CmdConfigHostnameTestFixture): argument validation (valid
  simple/FQDN, empty args, empty string, too-many args), set-new hostname,
  already-set no-op.
• End-to-end integration test (ConfigHostnameTest) that sets, verifies,
  and restores the hostname. The integration test reads the initial
  hostname with a "" default so setup tolerates platforms where the
  optional hostname field is unset and therefore omitted from the running
  config.

Unit tests (2026-06-15)

$ bazel build //fboss/cli/fboss2/test/config:cmd_config_test
INFO: Build completed successfully, 1549 total actions

$ ./bazel-bin/fboss/cli/fboss2/test/config/cmd_config_test --gtest_filter="*ConfigHostname*"
[==========] Running 3 tests from 1 test suite.
[ RUN      ] CmdConfigHostnameTestFixture.argValidation
[       OK ] CmdConfigHostnameTestFixture.argValidation (60 ms)
[ RUN      ] CmdConfigHostnameTestFixture.setNewHostname
[       OK ] CmdConfigHostnameTestFixture.setNewHostname (83 ms)
[ RUN      ] CmdConfigHostnameTestFixture.alreadySet
[       OK ] CmdConfigHostnameTestFixture.alreadySet (56 ms)
[==========] 3 tests from 1 test suite ran.
[  PASSED  ] 3 tests.

Full cmd_config_test suite (no regressions):

[==========] 274 tests from 25 test suites ran. (16065 ms total)
[  PASSED  ] 274 tests.

Review Findings

Pre-publication review (single-pass automated reviewer). One item raised —
unchecked *config.sw() dereference in CmdConfigHostname.cpp — reviewed
and dismissed: it is the established pattern across all fboss2 config
commands (e.g. CmdConfigArp.cpp, CmdConfigCopp.cpp); sw is reliably
present for a loaded agent config. No issues above threshold.

Presubmission Checks

• clang-format (--style=file --dry-run --Werror): pass on all added/modified C++ sources.
• pre-commit: pass (no violations on changed files).

[joseph5wu]

Consider to use CmdConfigSwitch to manage individual Switch Settings

[joseph5wu]

Please consider to use switch_state to verify the config result:
https://github.com/facebook/fboss/blob/main/fboss/agent/switch_state.thrift#L478

fboss2_integration_test already has the following utility function:
https://github.com/facebook/fboss/blob/main/fboss/cli/fboss2/test/integration_test/Fboss2IntegrationTest.h#L131

[vybhav-nexthop]

the issue with these fields is that they are optional, therefore we pass a default as the second argument , so if the config field is not present in the agent.conf, we return the default in our tests . Similar modifications were done in these PRs -> (#1297,
#1268).

[joseph5wu]

Please use https://github.com/facebook/fboss/blob/main/fboss/cli/fboss2/test/integration_test/Fboss2IntegrationTest.h#L131 to verify rather than introducing the new getSwConfigFieldOpt() function

[vybhav-nexthop]

Thanks for the review. Addressed all three points in the latest commit:

1. CmdConfigSwitch: Introduced CmdConfigSwitch as a new parent command and moved hostname under it as config switch hostname.
2. switch_state verification: Integration test now uses getSwitchSettingsField/verifySwitchSettingsField (reading from switchSettingsMap in switch_state) instead of reading from switch_config.
3. Removed getSwConfigFieldOpt: Dropped the getSwConfigField{Opt}() helpers from Fboss2IntegrationTest.h entirely.