Add solver source variable insertion

[jurei733]

Summary

• Add a Solver expression warning when referenced variables are not selected as source variables.
• Add a source-variable insert menu for selected Solver sources, with Material tooltip and accessible label.
• Move the Solver test section above the syntax help and cover the new behavior with focused tests.

Refs #164

Validation

• npx ng test ngx-coding-components --watch=false --browsers=ChromeHeadless --include=projects/ngx-coding-components/src/lib/var-coding/dialogs/edit-source-parameters-dialog.component.spec.ts
• npm run lint (existing max-len warning in generate-coding-dialog.component.spec.ts)
• npx ng build ngx-coding-components
• git diff --check
• Browser check on http://127.0.0.1:4200/