feat: migrate testing framework to Bun and update test configurations

- Updated GitHub Actions workflow to use Bun's test runner and coverage reporting.
- Added comprehensive testing documentation for the Gitea Mirror project.
- Refactored test scripts in package.json to align with Bun's testing commands.
- Created new test files for database, Gitea, GitHub, health, and mirroring APIs.
- Implemented mock functions for API tests to handle various scenarios and responses.
- Established a test setup file for consistent test environment configuration.

package.json

@@ -21,8 +21,9 @@
     "preview": "bunx --bun astro preview",
     "start": "bun dist/server/entry.mjs",
     "start:fresh": "bun run cleanup-db && bun run manage-db init && bun run update-db && bun dist/server/entry.mjs",
-    "test": "bunx --bun vitest run",
-    "test:watch": "bunx --bun vitest",
+    "test": "bun test",
+    "test:watch": "bun test --watch",
+    "test:coverage": "bun test --coverage",
     "astro": "bunx --bun astro"
   },
   "dependencies": {