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
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