[FEATURE]: Expose V2 session resume through the public API #46198

Feature hasn't been suggested before.\n\n- [x] I have verified this feature I'm about to request hasn't been suggested before.\n\n### Describe the enhancement you want to request\n\nExpose the existing V2 session resume operation through the public HTTP API and generated clients.\n\nProgrammatic clients need to continue a persisted session after a restart without appending a synthetic user message or importing private core services. The operation should keep the current session state and return the existing not-found and service-unavailable errors.\n\n#35443 covers broader plugin APIs, but does not request this HTTP continuation endpoint.