test(artifact): cover version lookup after deletion

[mikemikimike]

Please ensure you have read the contribution guide before creating a pull request.

Link to Issue or Description of Change

• Closes: Missing contract test coverage for GetArtifactVersion() #681

Problem:

The shared artifact service contract suite verifies that GetArtifactVersion returns fs.ErrNotExist for an empty service, a missing version, and a missing file, but it does not verify the behavior after an existing artifact is deleted.

Solution:

Extend the shared service suite with a post-deletion GetArtifactVersion assertion. The test runs through the existing in-memory and fake-GCS service implementations and requires an error that matches fs.ErrNotExist.

Testing Plan

Unit Tests:

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

Passed results:

• go test -mod=readonly -count=1 -shuffle=on ./artifact/... ./internal/artifact/...
• docker run ... golang:1.26.6 go test -race -mod=readonly -count=1 -shuffle=on work (all workspace packages passed; the Windows worktree was copied to a temporary Linux directory to normalize CRLF before running the repository's copyright-header test)
• go build -mod=readonly work
• go build -mod=readonly ./... in plugin/agentanalytics
• golangci-lint run --allow-parallel-runners --new-from-rev=HEAD
• git diff --check and gofmt

Known local limitations:

• go mod tidy -diff reports a CRLF/LF-only diff on the Windows worktree and does not modify any files.
• Full golangci-lint run reports pre-existing gofumpt issues in platform/*.go and plugin/agentanalytics/*.go; the changed file is clean under --new-from-rev=HEAD.

Manual End-to-End (E2E) Tests:

Not applicable: this is a backend contract test with no user-facing runtime change or external service setup.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly where needed for this small assertion.
• I have added tests that prove the behavior after deletion.
• New and existing unit tests pass locally in the relevant packages and workspace race run.
• Manual E2E testing is not applicable to this test-only change.
• No dependent changes are required.

Additional context

No production code or public API behavior is changed.