refactor(frontier): remove grpc-gateway and openapiv2 annotations (#444)

* refactor(frontier): remove grpc-gateway and openapiv2 annotations

Frontier no longer uses grpc-gateway. Remove all gateway-related
annotations from proto files:

- Remove `protoc-gen-openapiv2` import, swagger, operation, and field annotations
- Remove `google.api.http` annotations and imports
- Keep `google.api.field_behavior` and `validate` annotations
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: whoAbhishekSah