fix: fail build when plot options schemas are not generated

[tomivirkki]

Summary

• The chart plot options schemas are generated at build time from the chart model sources and shipped in the jar. When the source directory was missing the generator only logged a warning and skipped, so a build with tests skipped could publish a jar without schemas and the tool would report every chart type as unknown. The generator now fails the build in that case, and a missing resource at runtime is logged as an error.
• Added tests that check the generated resource is on the classpath, covers every supported chart type with descriptions, and is exposed through ChartAIController's tools, so a regression in generation or packaging fails the module's unit tests directly.

[vaadin-review-bot]

✅ Nothing to flag — the changes look good.

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
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

[ugur-vaadin]

Should we also target v25.3?

[tomivirkki]

Should we also target v25.3?

I was originally planning to exclude it from 25.3, but shouldn't hurt to have it there either. Let's keep the CP PR in draft for a while just to be sure it doesn't affect build