test(file-service): cover DatasetResource path validation

[Ma77Ball]

What changes were proposed in this PR?

• Add DatasetResourcePathSpec, a lightweight unit spec calling the companion validateAndNormalizeFilePathOrThrow directly.
• Cover the empty-path, normalize-to-null, absolute-path, and normalized-relative-path branches.

Any related issues, documentation, discussions?

Closes: #6700

How was this PR tested?

• Run: sbt "FileService/testOnly *DatasetResourcePathSpec", expect all 7 tests passing.
• Test-only change; no production code is modified.

Was this PR authored or co-authored using generative AI tooling?

Co-authored with Claude Opus 4.8 in compliance with ASF

[github-actions]

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[Copilot]

Pull request overview

This PR adds focused ScalaTest unit coverage for DatasetResource.validateAndNormalizeFilePathOrThrow, improving confidence in file-path validation behavior within the file-service without relying on the heavier DatasetResourceSpec integration harness.

Changes:

• Introduce a new lightweight spec that directly exercises validateAndNormalizeFilePathOrThrow.
• Add tests covering null/empty/whitespace inputs, normalization-to-null invalid traversal, absolute-path rejection, and relative-path normalization behavior.

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[aglinxinyuan]

LGTM!