feat: support file_data URI references in GcsArtifactService

[blueeye-040]

GcsArtifactService previously raised NotImplementedError when saving a types.Part with file_data. This PR adds full file_data URI support to match the existing InMemoryArtifactService behavior.

Link to Issue or Description of Change

• Closes: feat: Support file_data (URI references) in GcsArtifactService.save_artifact #5230

Problem:
GcsArtifactService._save_artifact raised NotImplementedError for file_data parts, while InMemoryArtifactService already handled them. This made GcsArtifactService unusable for any workflow that passes file URI references (e.g. gs:// or artifact:// URIs) as artifacts.

Solution:
Store file_data URI references as zero-byte GCS blobs with a file_uri custom metadata key — no content to upload, just the URI pointer. On load, check for that metadata key before downloading bytes and restore the original types.Part(file_data=...). Also switched _load_artifact to use bucket.get_blob() (which populates metadata) instead of bucket.blob() (which does not). artifact:// URIs are validated via artifact_util.parse_artifact_uri before saving.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Added 4 new tests in tests/unittests/artifacts/test_artifact_service.py:

• test_gcs_save_artifact_with_external_gcs_uri — roundtrip save/load with a gs:// URI
• test_gcs_save_artifact_with_artifact_ref_uri — roundtrip save/load with an artifact:// URI
• test_gcs_save_artifact_file_data_without_mime_type — file_data with no mime_type
• test_gcs_save_artifact_file_data_missing_uri_raises — empty file_uri raises InputValidationError

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[blueeye-040]

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