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.

src/tests/setup.bun.ts

@@ -0,0 +1,20 @@
+/**
+ * Bun test setup file
+ * This file is automatically loaded before running tests
+ */
+
+import { afterEach, beforeEach } from "bun:test";
+
+// Clean up after each test
+afterEach(() => {
+  // Add any cleanup logic here
+});
+
+// Setup before each test
+beforeEach(() => {
+  // Add any setup logic here
+});
+
+// Add DOM testing support if needed
+// import { DOMParser } from "linkedom";
+// global.DOMParser = DOMParser;