test(artifact): cover GetArtifactVersion

[mdabydeen]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: N/A
• Related: N/A

2. Or, if no issue exists, describe the change:

Problem:
GetArtifactVersion on the shared artifact service test suite had no direct test coverage for retrieving the latest version, an explicit version, or the not-found paths for a missing version/file.

Solution:
Add a testArtifactService_GetArtifactVersion sub-test to internal/artifact/tests/service_suite.go that saves three versions of an artifact and exercises GetArtifactVersion for the latest version, an explicit version, a nonexistent version, and a nonexistent file, asserting fs.ErrNotExist for the not-found cases. This runs against both the in-memory and mocked GCS implementations via the shared suite. Also extended the existing empty-service test with a GetArtifactVersion not-found case.

Testing Plan

Unit Tests:

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

ok  	google.golang.org/adk/v2/internal/artifact	0.421s

Manual End-to-End (E2E) Tests:

N/A — test-only change to the shared artifact service test suite.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

N/A