Validate GAR switch IDs at startup

Summary: Context

GAR policies encode the local switch identifier into a field of the link-bandwidth extended community. Today BGP discovers that the value does not fit only when a route reaches LbwExtCommunityAction::applyAction, where encodeValue terminates the process through an XCHECK.

Startup ordering

All three BGP entry points construct Config, load the policy bundle, and call Config::createPolicyManager before constructing or starting RIB. This diff validates at that common boundary. A bad switch/policy pair therefore raises BgpError during BGP startup, before any RIB thread or route processing begins. The runtime check remains only as an invariant backstop.

Which policies apply

The policy bundle is shared and may contain statements for other switch roles. Validating the whole bundle would reject an STSW because of an RTSW-only policy. getReferencedPolicyNames therefore seeds validation from the resolved static-peer and dynamic-peer configs, which already include peer-group inheritance and peer overrides, plus policies attached to originated IPv4/IPv6 networks.

Policy traversal

validateSwitchIdEncoding builds a name index and walks only the reachable policy graph. NEXT_POLICY edges are followed transitively, and a visited set handles repeated references and cycles. Missing root names remain the responsibility of the existing Config::verifyIfPoliciesExist validation.

Encoding check

For each reachable ENCODE_SWITCH_ID action, encoding_id selects a field position in the configured encoding scheme. This supports either rack-ID or plane-ID layouts without hard-coding a field name. BGP rejects missing switch IDs, incomplete schemes, invalid field indices, and values outside the selected unsigned bit width. The shared hostname parser consumes the complete trailing decimal run, so rtsw1024 is ID 1024 rather than 24.

Reviewed By: xiangxu1121

Differential Revision: D113616754

fbshipit-source-id: 25d4fa61c20293a04b6e957ae704e897a1acc6e2