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/vite.config.ts

@@ -8,4 +8,7 @@ export default defineConfig({
     strictPort: true,
   },
   clearScreen: false,
+  test: {
+    include: ['src/**/*.test.ts'],
+  },
 })