Push provider code and trigger a Cloud Build.
Accepts a tarball containing provider code and triggers a Cloud Build to create a container image. Returns a CalVer version for tracking.
Args: provider_id: Unique identifier for the provider. code: Tarball (tar.gz) containing provider source code. service: Provider service for build orchestration.
Returns: PushResponse with CalVer version and Cloud Build ID for tracking.
Raises: HTTPException: 400 if build creation fails.