Add solver test to derivation dialog

[jurei733]

Summary

• Adds a Solver-Test section to the Basis-/Ableitungsparameter dialog for SOLVER derivations.
• Lets users enter sample values for selected source variables and evaluate the current solver expression.
• Shows the derived result or clear validation/evaluation errors without saving the dialog.

Context

Related issue: #162

This intentionally does not close the issue automatically.

Validation

• npm run test:cc -- --include=projects/ngx-coding-components/src/lib/var-coding/dialogs/edit-source-parameters-dialog.component.spec.ts
• npm run test:cc
• npm run lint (passes with one pre-existing max-len warning in generate-coding-dialog.component.spec.ts:381)