refactor: eliminate unsafe type assertions from Group 2 test files (#8258)

## Summary
Improved type safety in test files by eliminating unsafe type assertions
and adopting official testing patterns. Reduced unsafe `as unknown as`
type assertions and eliminated all `null!` assertions.

## Changes
- **Adopted @pinia/testing patterns**
- Replaced manual Pinia store mocking with `createTestingPinia()` in
`useSelectionState.test.ts`
  - Eliminated ~120 lines of mock boilerplate
- Created `createMockSettingStore()` helper to replace duplicated store
mocks in `useCoreCommands.test.ts`

- **Eliminated unsafe null assertions**
- Created explicit `MockMaskEditorStore` interface with proper nullable
types in `useCanvasTools.test.ts`
- Replaced `null!` initializations with `null` and used `!` at point of
use or `?.` for optional chaining

- **Made partial mock intent explicit**
- Updated test utilities in `litegraphTestUtils.ts` to use explicit
`Partial<T>` typing
- Changed cast pattern from `as T` to `as Partial<T> as T` to show
incomplete mock intent
- Applied to `createMockLGraphNode()`, `createMockPositionable()`, and
`createMockLGraphGroup()`

- **Created centralized mock utilities** in
`src/utils/__tests__/litegraphTestUtils.ts`
- `createMockLGraphNode()`, `createMockPositionable()`,
`createMockLGraphGroup()`, `createMockSubgraphNode()`
  - Updated 8+ test files to use centralized utilities
- Used union types `Partial<T> | Record<string, unknown>` for flexible
mock creation

## Results
- ✅ 0 typecheck errors
- ✅ 0 lint errors  
- ✅ All tests passing in modified files
- ✅ Eliminated all `null!` assertions
- ✅ Reduced unsafe double-cast patterns significantly

## Files Modified (18)
- `src/components/graph/SelectionToolbox.test.ts`
-
`src/components/graph/selectionToolbox/{BypassButton,ColorPickerButton,ExecuteButton}.test.ts`
- `src/components/sidebar/tabs/queue/ResultGallery.test.ts`
- `src/composables/canvas/useSelectedLiteGraphItems.test.ts`
- `src/composables/graph/{useGraphHierarchy,useSelectionState}.test.ts`
-
`src/composables/maskeditor/{useCanvasHistory,useCanvasManager,useCanvasTools,useCanvasTransform}.test.ts`
- `src/composables/node/{useNodePricing,useWatchWidget}.test.ts`
- `src/composables/{useBrowserTabTitle,useCoreCommands}.test.ts`
- `src/utils/__tests__/litegraphTestUtils.ts`

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8258-refactor-eliminate-unsafe-type-assertions-from-Group-2-test-files-2f16d73d365081549c65fd546cc7c765)
by [Unito](https://www.unito.io)

---------

Co-authored-by: GitHub Action <action@github.com>
Co-authored-by: Alexander Brown <drjkl@comfy.org>
Co-authored-by: Amp <amp@ampcode.com>
Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>
Co-authored-by: AustinMroz <austin@comfy.org>
Co-authored-by: Christian Byrne <cbyrne@comfy.org>
Co-authored-by: Benjamin Lu <benjaminlu1107@gmail.com>

src/composables/functional/useChainCallback.ts

@@ -1,19 +1,3 @@
-/**
- * Shorthand for {@link Parameters} of optional callbacks.
- *
- * @example
- * ```ts
- * const { onClick } = CustomClass.prototype
- * CustomClass.prototype.onClick = function (...args: CallbackParams<typeof onClick>) {
- *   const r = onClick?.apply(this, args)
- *   // ...
- *   return r
- * }
- * ```
- */
-export type CallbackParams<T extends ((...args: any) => any) | undefined> =
-  Parameters<Exclude<T, undefined>>
-
 /**
  * Chain multiple callbacks together.
  *
@@ -21,15 +5,21 @@ export type CallbackParams<T extends ((...args: any) => any) | undefined> =
  * @param callbacks - The callbacks to chain.
  * @returns A new callback that chains the original callback with the callbacks.
  */
-export const useChainCallback = <
-  O,
-  T extends (this: O, ...args: any[]) => void
->(
+export function useChainCallback<O, T>(
   originalCallback: T | undefined,
-  ...callbacks: ((this: O, ...args: Parameters<T>) => void)[]
-) => {
-  return function (this: O, ...args: Parameters<T>) {
-    originalCallback?.call(this, ...args)
-    for (const callback of callbacks) callback.call(this, ...args)
-  }
+  ...callbacks: NonNullable<T> extends (this: O, ...args: infer P) => unknown
+    ? ((this: O, ...args: P) => void)[]
+    : never
+) {
+  type Args = NonNullable<T> extends (...args: infer P) => unknown ? P : never
+  type Ret = NonNullable<T> extends (...args: unknown[]) => infer R ? R : never
+
+  return function (this: O, ...args: Args) {
+    if (typeof originalCallback === 'function') {
+      ;(originalCallback as (this: O, ...args: Args) => Ret).call(this, ...args)
+    }
+    for (const callback of callbacks) {
+      callback.call(this, ...args)
+    }
+  } as (this: O, ...args: Args) => Ret
 }