Improve route form interaction with upfront required-field validation

[Copilot]

This issue targets user experience/user interaction in the Routes workflow. The form currently allows users to attempt submission without clear, immediate guidance on missing required inputs.

• What changed

  • Added a shared readiness predicate for route submission (route_ready_for_submit).
  • Added an upfront validation gate in handle_submit to block invalid submissions before save calls.
  • Improved validation copy to explicitly call out required fields: external path, internal path, HTTP method(s), and backend target.

• User-facing interaction improvement

  • Users now get direct, actionable feedback when trying to save an incomplete route, instead of a later failure path.
  • Guidance text is aligned with field names to reduce ambiguity.

• Code-level structure

  • Centralized submit-readiness logic into a dedicated function.
  • Added focused unit coverage for readiness behavior (missing backend, invalid paths/methods, valid route).

if !route_ready_for_submit(&final_route) {
    set_validation_error.set(Some(
        "Please fill in both external and internal paths, choose at least one HTTP method, and add a valid backend target before saving."
            .to_string(),
    ));
    return;
}