Validate Routes in Test Cases throwing an error if route is not specified when required

[goetas]

Validate Routes in Test Cases

Discovered in https://github.com/getyourguide/routes/pull/3029 , see (https://github.com/getyourguide/routes/pull/3029#discussion_r2704499904)

This pull request introduces validation to ensure that when simple routing is requested (with wantMatch set to false), a route must be provided in the test cases. Without a defined route, the parser now returns an explicit error.

Changes include:

• Addition of an error constant ErrRouteRequiredForSimpleRouting to signal missing required routes.
• Parser update to check that test cases either define a rewrite, redirect, delegate, or at least one route when wantMatch is false.
• A new test file with test cases illustrating missing route scenarios.
• Unit tests validating that missing routes cause parsing to fail appropriately.

This improves the robustness of configuration validation by enforcing necessary route definitions in test cases.