refactor: consume common and ai-gateway protos from BSR packages

Remove buf.build/redpandadata/common and buf.build/redpandadata/ai-gateway
as explicit inputs from buf.gen.backend.yaml and buf.gen.frontend.yaml.

Backend: common types are consumed as pre-built Go packages from the BSR
SDK (buf.build/gen/go/redpandadata/common/protocolbuffers/go), matching
the pattern used by cloudv2. This eliminates the policymaterializer
codegen issue entirely.

Frontend: common and ai-gateway types are consumed from BSR npm packages
(@buf/redpandadata_common.bufbuild_es, @buf/redpandadata_ai-gateway.bufbuild_es,
@buf/redpandadata_ai-gateway.connectrpc_query-es). Only console's own
dataplane protos are generated locally.

Author: c-julin