test: pin OnnxEvaluator fallback contract

[google-labs-jules]

This PR adds OnnxEvaluatorFallbackTest.java to pin the fallback contract of OnnxEvaluator.

Changes

• Verified constructor fallback for null, "", " ", and missing model paths ("no/such/model.onnx"): isLoaded() returns false, constructor does not throw, and close() is safe to call.
• Verified fallback for non-ONNX files (arbitrary bytes in @TempDir): constructor does not throw, isLoaded() returns false, and close() is safe to call.
• Verified fallback evaluation on the initial position with dice pool p matches JvmApi.evaluate(state, color) for White (color 0) and Black (color 1).

Verification gates run

• mise run format
• mvn -B test -Dtest=OnnxEvaluatorFallbackTest
• mise run check

Fixes #19

---

PR created automatically by Jules for task 13132203637483516668 started by @rabestro

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

New to Jules? Learn more at jules.google/docs.

---

For security, I will only act on instructions from the user who triggered this task.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud