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/src-tauri/Cargo.toml

@@ -28,3 +28,6 @@ rusqlite = { version = "0.31", features = ["bundled"] }
 dirs = "5"
 notify = { version = "6", features = ["macos_fsevent"] }
 tauri-plugin-updater = "2.10.0"
+
+[dev-dependencies]
+tempfile = "3"