feat(artifacts): add save_media_frames() for batched media persistence

[copybara-service]

feat(artifacts): add save_media_frames() for batched media persistence

Introduces save_media_frames() on the artifact service layer. It accepts
frames: list[MediaFrame] -- a batch of media blobs each paired with its
capture timestamp -- and persists them as a single artifact, so a stream of
realtime video or image frames can be stored in one call instead of one
artifact per frame.

Implemented across the service hierarchy:

• MediaFrame: new model pairing a types.Blob with its capture timestamp,
  exported from google.adk.artifacts.
• BaseArtifactService.save_media_frames(): new non-abstract method that
  raises NotImplementedError by default, so existing third-party artifact
  services keep working without modification.
• InMemoryArtifactService, FileArtifactService, GcsArtifactService:
  concrete implementations.
• _ForwardingArtifactService and PerAgentFileArtifactService: delegate to
  the wrapped service.

This is a storage-layer change with no dependency on the live flow or the
cache manager, and it adds no new call sites; the caller that uses it ships
separately. It is not quite purely additive: GcsArtifactService's
delete_artifact() now has media frames to clean up, which it does from the
listing it already performs rather than by issuing an additional one.

Also documents the new method in the artifact service guide.

Testing:

• tests/unittests/artifacts and
  tests/unittests/tools/test_forwarding_artifact_service.py: 999 passed.
• New coverage: round-trip persistence and retrieval per backend, empty-frame
  handling, timestamp ordering, and NotImplementedError from the base class.