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/utils/__tests__/litegraphTestUtils.ts

@@ -0,0 +1,86 @@
+import type { Positionable } from '@/lib/litegraph/src/interfaces'
+import { Rectangle } from '@/lib/litegraph/src/infrastructure/Rectangle'
+import type {
+  LGraphCanvas,
+  LGraphGroup,
+  LGraphNode
+} from '@/lib/litegraph/src/litegraph'
+import { LGraphEventMode } from '@/lib/litegraph/src/litegraph'
+import { vi } from 'vitest'
+
+/**
+ * Creates a mock LGraphNode with minimal required properties
+ */
+export function createMockLGraphNode(
+  overrides: Partial<LGraphNode> | Record<string, unknown> = {}
+): LGraphNode {
+  const partial: Partial<LGraphNode> = {
+    id: 1,
+    pos: [0, 0],
+    size: [100, 100],
+    title: 'Test Node',
+    mode: LGraphEventMode.ALWAYS,
+    ...(overrides as Partial<LGraphNode>)
+  }
+  return partial as Partial<LGraphNode> as LGraphNode
+}
+
+/**
+ * Creates a mock Positionable object
+ */
+export function createMockPositionable(
+  overrides: Partial<Positionable> = {}
+): Positionable {
+  const partial: Partial<Positionable> = {
+    id: 1,
+    pos: [0, 0],
+    ...overrides
+  }
+  return partial as Partial<Positionable> as Positionable
+}
+
+/**
+ * Creates a mock LGraphGroup with minimal required properties
+ */
+export function createMockLGraphGroup(
+  overrides: Partial<LGraphGroup> = {}
+): LGraphGroup {
+  const partial: Partial<LGraphGroup> = {
+    id: 1,
+    pos: [0, 0],
+    boundingRect: new Rectangle(0, 0, 100, 100),
+    ...overrides
+  }
+  return partial as Partial<LGraphGroup> as LGraphGroup
+}
+
+/**
+ * Creates a mock SubgraphNode with sub-nodes
+ */
+export function createMockSubgraphNode(
+  subNodes: LGraphNode[],
+  overrides: Partial<LGraphNode> | Record<string, unknown> = {}
+): LGraphNode {
+  const baseNode = createMockLGraphNode(overrides)
+  return Object.assign(baseNode, {
+    isSubgraphNode: () => true,
+    subgraph: {
+      nodes: subNodes
+    }
+  })
+}
+
+/**
+ * Creates a mock LGraphCanvas with minimal required properties for testing
+ */
+export function createMockCanvas(
+  overrides: Partial<LGraphCanvas> = {}
+): LGraphCanvas {
+  return {
+    setDirty: vi.fn(),
+    state: {
+      selectionChanged: false
+    },
+    ...overrides
+  } as LGraphCanvas
+}