modelerrors: add NewContextOverflowError constructor

Callers were constructing &ContextOverflowError{Underlying: err}
directly. Adding a constructor improves encapsulation and allows
future field additions without breaking callers. Update
runtime/fallback.go to use the constructor.

Assisted-By: docker-agent

Author: dgageot

pkg/modelerrors/modelerrors.go

@@ -84,6 +84,13 @@ type ContextOverflowError struct {
 	Underlying error
 }
 
+// NewContextOverflowError creates a ContextOverflowError wrapping the given
+// underlying error. Use this constructor rather than building the struct
+// directly so that future field additions don't break callers.
+func NewContextOverflowError(underlying error) *ContextOverflowError {
+	return &ContextOverflowError{Underlying: underlying}
+}
+
 func (e *ContextOverflowError) Error() string {
 	if e.Underlying == nil {
 		return "context window overflow"

pkg/runtime/fallback.go

@@ -326,7 +326,7 @@ func (r *LocalRuntime) tryModelWithFallback(
 	if lastErr != nil {
 		wrapped := fmt.Errorf("all models failed: %w", lastErr)
 		if modelerrors.IsContextOverflowError(lastErr) {
-			return streamResult{}, nil, &modelerrors.ContextOverflowError{Underlying: wrapped}
+			return streamResult{}, nil, modelerrors.NewContextOverflowError(wrapped)
 		}
 		return streamResult{}, nil, wrapped
 	}