refactor: merge test-utils package into core (#3200)

* refactor: merge test-utils package into core

Consolidate the standalone @qwen-code/qwen-code-test-utils package
into packages/core/src/test-utils/, eliminating the need for a
separate package that only provided createTmpDir, cleanupTmpDir,
and FileSystemStructure type.

Changes:
- Move file-system-test-helpers.ts into core/src/test-utils/
- Re-export from core's test-utils index
- Update 3 core test files to use relative imports
- Update cli useAtCompletion test to import from @qwen-code/qwen-code-core
- Remove test-utils devDependency from core and cli package.json
- Delete packages/test-utils/ directory

All affected tests pass (fileSearch, crawler, ignore, useAtCompletion).

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

* fix: remove deleted test-utils from build order

The test-utils package was merged into core but the build script still
tried to build it separately, causing CI failures.

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

---------

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

packages/cli/package.json

@@ -78,8 +78,6 @@
   },
   "devDependencies": {
     "@babel/runtime": "^7.27.6",
-    "@google/gemini-cli-test-utils": "file:../test-utils",
-    "@qwen-code/qwen-code-test-utils": "file:../test-utils",
     "@testing-library/react": "^16.3.0",
     "@types/archiver": "^6.0.3",
     "@types/command-exists": "^1.2.3",