test(workflow-operator): add ImageUtilitySpec for encodeImageToHTML

[KYinXu]

What changes were proposed in this PR?

Pin the Python snippet emitted by ImageUtility.encodeImageToHTML() — the helper that visualization operators splice into generated UDF code to base64-encode binary image data into an tag. Any drift in the f-string template or the error fallback would silently break image-rendering operators (e.g. Word Cloud). No production-code changes. Negative cases are covered implicitly: removing any pinned substring fails the corresponding assertion.

Spec	Source Class	Tests
ImageUtilitySpec	ImageUtility	8

Spec file name follows the <srcClassName>Spec.scala one-to-one convention.

Behavior pinned

Surface	Contract
encodeImageToHTML	returns a non-empty String
Snippet imports base64	substring import base64 appears
Snippet calls base64.b64encode(binary_image_data)	substring appears verbatim
Snippet decodes to UTF-8	substring .decode("utf-8") appears
Snippet emits an <img> tag with data:image;base64,...	substring data:image;base64,{encoded_image_str} appears (verifies the f-string template)
Snippet handles binary-decode failure via try/except	substrings except Exception and Binary input is not valid both appear
Snippet contains a top-level html = assignment	substring html = f appears so downstream code can reference html
Determinism	two calls return the exact same string

The harness pins via contains on canonical substrings — resilient to incidental whitespace changes from stripMargin while still catching real template drift.

Any related issues, documentation, discussions?

Closes #5665

How was this PR tested?

Unit tests were added here, tested through local verification:

• sbt "WorkflowOperator/testOnly org.apache.texera.amber.operator.visualization.ImageUtilitySpec" - All 8 tests passed
• sbt scalafmtCheckAll -- clean
• sbt scalafixAll --check No lint errors

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

Generated-by: Composer 2.5 Fast

[github-actions]

👋 Thanks for your first contribution to Texera, @KYinXu!

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