Add typed routes, params, and response body to test app

[viniciusdacal]

What

The test app should provide full type safety for routes, request params/body/headers, and response bodies:

• Typed route strings — autocomplete for registered routes (app.get('/users/:id') type-checked)
• Typed request params/body/headers per route — params: { id: string } inferred from route schema
• Typed response body — res.body narrows via res.ok discriminated union
• Typed request headers per route — headers typed from route's header schema

Blocked by

• Compiler implementation — these features require the compiler to generate route type information from module/router definitions

Context

Listed as "Blocked by Compiler (out of scope)" in the testing implementation plan (plans/vertz-testing-implementation.md, lines 9-16). Also described in the testing design doc (plans/vertz-testing-design.md).