test(v2): add vitest and cargo tests for sdk-messages, agent-tree, session, ctx

Frontend (vitest):
- sdk-messages.test.ts: adaptSDKMessage() for all 9 message types
- agent-tree.test.ts: buildAgentTree(), countTreeNodes(), subtreeCost()
- vite.config.ts: vitest test config (src/**/*.test.ts)
- package.json: vitest ^4.0.18 dev dep, "test" script

Backend (cargo):
- session.rs: SessionDb CRUD tests (sessions, SSH, settings, layout) with tempfile
- ctx.rs: CtxDb error handling tests with missing database
- Cargo.toml: tempfile 3 dev dependency

v2/package.json

@@ -10,7 +10,8 @@
     "check": "svelte-check --tsconfig ./tsconfig.app.json && tsc -p tsconfig.node.json",
     "tauri": "cargo tauri",
     "tauri:dev": "cargo tauri dev",
-    "tauri:build": "cargo tauri build"
+    "tauri:build": "cargo tauri build",
+    "test": "vitest run"
   },
   "devDependencies": {
     "@sveltejs/vite-plugin-svelte": "^6.2.1",
@@ -19,7 +20,8 @@
     "svelte": "^5.45.2",
     "svelte-check": "^4.3.4",
     "typescript": "~5.9.3",
-    "vite": "^7.3.1"
+    "vite": "^7.3.1",
+    "vitest": "^4.0.18"
   },
   "dependencies": {
     "@tauri-apps/api": "^2.10.1",