[Nexthop][fboss2-dev] fboss2 config/delete protocol bgp neighbor commands - #1391

Follow-up cleanups to the BGP-aware config session infra (facebook#1344):

- Collapse the BGP_RESTART action level into AGENT_WARMBOOT (bgpd has no hitless
  reload; its restart already runs the agent-warmboot code path).
- Introduce a single ConfigDomain descriptor + configDomains() and shared
  per-domain helpers so commit(), rollback() and `config session diff` handle
  the agent and BGP domains uniformly (private DiffDomain removed).
- Make the agent skip-when-unchanged like BGP: a commit whose staged config
  equals what is already promoted is a true no-op (no git revision, no symlink
  churn, no reloadConfig()/bgpd restart). Change detection is semantic (compare
  the deserialized thrift structs), so formatting-only diffs don't count.
- Consolidate `config session clear` onto a static stagedSessionFilePaths()
  and reuse ConfigSession::readStagedContent() in diff.
- Make ConfigSession::saveConfig(service, level) generic over the service and
  reduce saveBgpConfig() to a thin wrapper.
- Keep the heavy generated thrift headers out of ConfigSession.h: use the
  *_types_fwd.h forward-declaration headers, hold agentConfig_/bgpConfig_ by
  std::unique_ptr, and drop the configLoaded_/bgpConfigLoaded_ bools
  (null == not loaded).
- clang-tidy: use auto for the SimpleJSONSerializer template-cast results.

Built fboss2-dev + the config unit tests; config-session/commit/diff/BGP/clear
tests pass. Verified the agent no-op behaviour live on test switches.

Add `fboss2-dev config protocol bgp global <attr> <value>` on top of the
BGP-aware ConfigSession (base PR). Edits the typed bgp::thrift::BgpConfig via
ConfigSession::getBgpConfig()/saveBgpConfig() -- the whole-config,
scope-agnostic typed API (no global/peer/peer-group special-casing in
ConfigSession).

- Collapse the 10 per-attribute global command classes into one dispatcher;
  reject cluster-id (no BgpConfig field) instead of writing dead config; bound
  switch-limit / max_golden_vips so out-of-range values aren't truncated.
- Integration tests: ConfigBgpGlobalTest (each attr set+commit, verified in the
  promoted /etc/coop/bgpcpp/bgpcpp.conf) and ConfigBgpSessionTest
  (clear/diff/commit/rollback + a no-op-restart regression), sharing
  ConfigBgpTestBase.

Test Plan:
- bazel test //fboss/cli/fboss2/test/config:cmd_config_test
- bazel build //fboss/cli/fboss2/test/integration_test:fboss2_integration_test
- fboss2_integration_test on a DUT with bgp_pp active: ConfigBgpSessionTest
  6/6 pass (clear, diff, commit-restarts-bgp_pp, rollback-restores-config, and
  the unchanged-config does-not-restart case); agent-session regression
  (ConfigInterfaceMtuTest) passes.

Adds `config protocol bgp neighbor <ip-address> [<attribute> <value> ...]`
and `delete protocol bgp neighbor <ip-address>` — the first per-peer BGP
config family, beneath the `protocol bgp` grouping node.

- neighbor dispatcher keyed by peer address, writing bgp_config.BgpPeer
  through the typed ConfigSession. Attributes cover the peer identity
  (remote/local ASN, description, peer-tag, peer-group), policy bindings
  (ingress/egress policy), session tunables (timers, route limits,
  next-hop, add-path, RR client, and the rest of the documented set), and
  the sheet-documented attributes bgpd does not support, which are
  rejected with an explanatory message rather than staged.
- delete removes the peer by address.
- ConfigBgpTestBase restores the committed BGP config after every test, so
  a failed run cannot leave a device carrying test config; commit-path
  integration tests verify each value through bgpd's own getRunningConfig
  RPC rather than only the staged file.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>