Harden diagnostic handling and warning presentation

internal/framework/contracts/correction.go

@@ -126,6 +126,9 @@ func (candidate ModelCandidate) Validate() error {
 }
 
 func ValidateValidationResult(result ValidationResult) error {
+	if err := ValidateProducerDiagnostics(result.Diagnostics); err != nil {
+		return fmt.Errorf("validation diagnostics: %w", err)
+	}
 	if !result.Approved && result.ReasonCode == "" {
 		return errors.New("validation rejection reason code must not be empty")
 	}