proto: add transcript service definition (#2309)

* proto: add transcript service and regenerate protobuf/openapi definitions

Adds the transcript proto definition for the dataplane v1alpha3 API
and regenerates all protobuf (backend Go, frontend TS) and OpenAPI outputs.

* chore(proto): pin ai-gateway version in buf.gen.frontend.yaml and regenerate

Pin ai-gateway to commit 0d2811b8 to prevent unrelated proto changes
from leaking into PRs that only need to regenerate dataplane protos.

* fix: revert buf.gen.frontend.yaml to match master's BSR package approach

Remove buf.build/redpandadata/common and buf.build/redpandadata/ai-gateway
module inputs from buf.gen.frontend.yaml. These types are now consumed
from pre-built BSR npm packages per PR #2324, not generated locally.

Author: weeco