refactor: improve TypeScript patterns in test files (Group 1/8) (#8253)

## Summary
Improves type safety in test files by replacing unsafe type patterns
with proper TypeScript idioms.

## Changes
- Define typed `TestWindow` interface extending `Window` for Playwright
tests with custom properties
- Use `Partial<HTMLElement>` with single type assertion for DOM element
mocks
- Remove redundant type imports
- Fix `console.log` → `console.warn` in test fixture

## Files Changed
16 test files across browser_tests, packages, and src/components

## Test Plan
- ✅ `pnpm typecheck` passes
- ✅ No new `any` types introduced
- ✅ All pre-commit hooks pass

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8253-refactor-improve-TypeScript-patterns-in-test-files-Group-1-8-2f16d73d365081548f9ece7bcf0525ee)
by [Unito](https://www.unito.io)

---------

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

src/base/common/async.ts

@@ -14,6 +14,7 @@ interface IdleDeadline {
 interface IDisposable {
   dispose(): void
 }
+type GlobalWindow = typeof globalThis
 
 /**
  * Internal implementation function that handles the actual scheduling logic.
@@ -21,7 +22,7 @@ interface IDisposable {
  * or fall back to setTimeout-based implementation.
  */
 let _runWhenIdle: (
-  targetWindow: any,
+  targetWindow: GlobalWindow,
   callback: (idle: IdleDeadline) => void,
   timeout?: number
 ) => IDisposable
@@ -37,7 +38,7 @@ export let runWhenGlobalIdle: (
 
   // Self-invoking function to set up the idle callback implementation
 ;(function () {
-  const safeGlobal: any = globalThis
+  const safeGlobal: GlobalWindow = globalThis as GlobalWindow
 
   if (
     typeof safeGlobal.requestIdleCallback !== 'function' ||