refactor(forms): remove Mutable from signal forms public API (angular#64436)

The `Mutable` type is only used internally in the `addDefaultField` function, so it can be avoided to be publicly exposed.

PR Close angular#64436

Author: cexbrayat

goldens/public-api/forms/signals/index.api.md

@@ -353,11 +353,6 @@ export class MinValidationError extends _NgValidationError {
     readonly min: number;
 }
 
-// @public
-export type Mutable<T> = {
-    -readonly [P in keyof T]: T[P];
-};
-
 // @public
 export const NgValidationError: abstract new () => NgValidationError;
 

packages/forms/signals/src/api/types.ts

@@ -16,16 +16,6 @@ import type {ValidationError, WithOptionalField, WithoutField} from './validatio
  */
 declare const ɵɵTYPE: unique symbol;
 
-/**
- * Creates a type based on the given type T, but with all readonly properties made writable.
- * @template T The type to create a mutable version of.
- *
- * @experimental 21.0.0
- */
-export type Mutable<T> = {
-  -readonly [P in keyof T]: T[P];
-};
-
 /**
  * A type that represents either a single value of type `T` or a readonly array of `T`.
  * @template T The type of the value(s).

packages/forms/signals/src/field/validation.ts

@@ -6,8 +6,8 @@
  * found in the LICENSE file at https://angular.dev/license
  */
 
-import {computed, Signal} from '@angular/core';
-import type {FieldTree, Mutable, TreeValidationResult, ValidationResult} from '../api/types';
+import {computed, Signal, ɵWritable} from '@angular/core';
+import type {FieldTree, TreeValidationResult, ValidationResult} from '../api/types';
 import type {ValidationError, WithOptionalField} from '../api/validation_errors';
 import {isArray} from '../util/type_guards';
 import type {FieldNode} from './node';
@@ -371,10 +371,10 @@ export function addDefaultField<E extends ValidationError>(
 ): ValidationResult<E> {
   if (isArray(errors)) {
     for (const error of errors) {
-      (error as Mutable<ValidationError>).field ??= field;
+      (error as ɵWritable<ValidationError>).field ??= field;
     }
   } else if (errors) {
-    (errors as Mutable<ValidationError>).field ??= field;
+    (errors as ɵWritable<ValidationError>).field ??= field;
   }
   return errors as ValidationResult<E>;
 }