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.

[adk-bot]

I have analyzed Pull Request #5914, which addresses a key bug in the ADK repository where the inline_data.display_name property of binary artifacts was discarded on save/load cycles in disk and cloud storage services.

I have compiled the comprehensive analysis into a premium PR Analysis Report artifact. Please inspect the report directly here:

📖 analysis_report.md

Key Summary of the Analysis

• Status & Target: This PR addresses Issue #5833, resolving a bug where both FileArtifactService and GcsArtifactService lost the original file display names on save/load pathways (unlike InMemoryArtifactService).
• Compliance Gate Passed: The contributor CLA check completed successfully on GitHub (SUCCESS state in action telemetry).
• Recommendation: Approve with Nits. The code is exceptionally clean, covers all edge cases gracefully, introduces robust unit tests, and avoids exposing framework-internal storage keys in custom_metadata structures.

Open Decision Points & Refinements (Nits)

I recommend asking the contributor to address the following minor styling nits so the codebase maintains strict style alignment:

1. Replace deprecated type-hints: Replace Optional[X] annotations with modern Pep-604 X | None unions across the modified sections:
   • display_name: Optional[str] in the FileArtifactVersion model should be typed as str | None.
   • _write_metadata's parameter display_name: Optional[str] in file_artifact_service.py should become str | None.
   • _user_metadata's input param metadata: Optional[dict[str, Any]] in gcs_artifact_service.py should become dict[str, Any] | None.

[rohityan]

Hi @he-yufeng , Thank you for your contribution! We appreciate you taking the time to submit this pull request.