Identify built-in generation preparation failures

internal/generation/coordinator.go

@@ -85,6 +85,12 @@ func (c *Coordinator) GenerateWithEvidence(
 	return c.generate(ctx, req, true)
 }
 
+// PreparationError identifies a built-in request preparation failure.
+type PreparationError struct{ Cause error }
+
+func (e *PreparationError) Error() string { return e.Cause.Error() }
+func (e *PreparationError) Unwrap() error { return e.Cause }
+
 func (c *Coordinator) generate(
 	ctx context.Context,
 	req domain.GenerateRequest,
@@ -95,7 +101,7 @@ func (c *Coordinator) generate(
 	}
 	prepared, err := c.builtIn.PrepareGeneration(req)
 	if err != nil {
-		return nil, err, GenerationEvidence{}
+		return nil, &PreparationError{Cause: err}, GenerationEvidence{PreparationFailed: true}
 	}
 
 	evidence := GenerationEvidence{}