feat(tests): add comprehensive happy path tests for session lifecycle

- Implemented session cleanup tests to ensure proper handling of session completions and cleanup operations.
- Added session initialization tests to verify session creation and observation queuing on first tool use.
- Created session summary tests to validate summary generation from conversation context upon session pause or stop.
- Developed integration tests to cover the full observation lifecycle, including context injection, observation queuing, and error recovery.
- Introduced reusable mock factories and scenarios for consistent testing across different test files.

package.json

@@ -31,7 +31,7 @@
   },
   "scripts": {
     "build": "node scripts/build-hooks.js",
-    "test": "node --test tests/",
+    "test": "vitest",
     "test:parser": "npx tsx src/sdk/parser.test.ts",
     "test:context": "echo '{\"session_id\":\"test-'$(date +%s)'\",\"cwd\":\"'$(pwd)'\",\"source\":\"startup\"}' | node plugin/scripts/context-hook.js 2>/dev/null",
     "test:context:verbose": "echo '{\"session_id\":\"test-'$(date +%s)'\",\"cwd\":\"'$(pwd)'\",\"source\":\"startup\"}' | node plugin/scripts/context-hook.js",
@@ -67,6 +67,7 @@
     "@types/react-dom": "^18.3.0",
     "esbuild": "^0.25.12",
     "tsx": "^4.20.6",
-    "typescript": "^5.3.0"
+    "typescript": "^5.3.0",
+    "vitest": "^4.0.15"
   }
 }