fix(artifacts): preserve inline display names

[he-yufeng]

Summary

FileArtifactService and GcsArtifactService currently preserve the bytes and MIME type of inline_data artifacts, but drop inline_data.display_name on load. That leaves downstream converters without the user-facing filename even when the caller provided one.

This stores the display name alongside the artifact payload and restores it when loading binary artifacts:

• file-backed artifacts persist it in the per-version metadata JSON
• GCS-backed artifacts store it in an internal blob metadata key
• user-supplied custom_metadata remains unchanged when artifact version metadata is listed or fetched

Fixes #5833.

Tested

• PYTHONPATH=src python -m pytest tests/unittests/artifacts/test_artifact_service.py::test_save_load_preserves_inline_data_display_name -q
• PYTHONPATH=src python -m py_compile src/google/adk/artifacts/file_artifact_service.py src/google/adk/artifacts/gcs_artifact_service.py tests/unittests/artifacts/test_artifact_service.py
• python -m pyink --check src/google/adk/artifacts/file_artifact_service.py src/google/adk/artifacts/gcs_artifact_service.py tests/unittests/artifacts/test_artifact_service.py
• git diff --check

I also ran the full artifact-service test file locally with PYTHONPATH=src. It passed all GCS/in-memory/display-name cases, with two existing Windows-only failures in tests that parse file:///C:/... URIs back with Path(unquote(parsed.path)), producing /C:/...; those are unrelated to this change.

pre-commit run --files ... passed the format/import hooks, but the two repository-local bash hooks could not run on this Windows checkout because /bin/bash is unavailable.