Add comprehensive test framework with vitest and TDD tests

- Set up vitest test framework with proper configuration
- Added test scripts to package.json (test, test:watch, test:ui)
- Created comprehensive SSRF validation tests (30 tests)
  - Tests for protocol validation (HTTP/HTTPS only)
  - Private IP blocking (127.x, 10.x, 172.16-31.x, 192.168.x, 169.254.x)
  - Kubernetes service DNS blocking (.svc, .cluster.local, etc.)
  - URL length validation (max 2048 chars)
  - DNS resolution error handling
  - Edge cases with ports, query params, userinfo
- Created cache service tests (19 tests)
  - Cache hit/miss operations
  - Deterministic key generation
  - TTL expiry behavior
  - Size limits and LRU eviction
  - Cache bypass logic
  - Statistics tracking
- Created integration test suite (marked as skip for CI)
  - Health endpoint tests
  - Playground endpoint tests with rate limiting
  - Authentication tests for screenshot endpoints
  - CORS header validation
  - Error handling and security headers
- All unit tests pass (49 total tests)
- Following strict Red/Green TDD methodology

vitest.config.ts

@@ -0,0 +1,21 @@
+import { defineConfig } from 'vitest/config'
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: 'node',
+    include: ['src/**/*.test.ts'],
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'html'],
+      exclude: [
+        'node_modules/',
+        'dist/',
+        '**/*.d.ts',
+        '**/*.config.*',
+      ]
+    },
+    testTimeout: 10000,
+    hookTimeout: 10000
+  },
+})