[Feature]: Add a fallback/catch-all route to Refit.Testing

[glennawatson]

Is your feature request related to a problem?

With Refit.Testing's StubHttp, every route needs an exact match on method, path, query, headers, etc. For tests that do not care about the exact request and just want a default response for anything not otherwise stubbed, that is more setup than the test needs. This was raised on PR #2195.

Describe the solution you'd like

A catch-all/fallback matcher on StubHttp's route table that responds to any request not matched by a more specific route, so a test can drop in one default reply instead of stubbing every route by hand. It should stay in Refit.Testing's flat, collection-initializer style rather than adding a fluent API, in line with the general goal of a mock framework that is perf/alloc-focused, easy to use, and test-framework agnostic.

Describe alternatives you've considered

A catch-all is already possible today with new RouteMatcher { Template = "*", Reusable = true }, but it's a workaround, not a real feature:

• It only works if it's the last entry in the table - reusable routes are matched in declaration order, so a "*" route earlier in the list shadows every reusable route after it.
• Nothing in the Route factory hints that this combination acts as a fallback.

Suggestions on how to achieve the feature

Add a Route.Fallback() factory (alongside Route.Get, Route.Post, etc.) that returns a RouteMatcher matching any method/path, and give StubHttp a third priority tier so fallback routes are always tried last - after one-shot and reusable routes - regardless of where they are declared in the table:

var http = new StubHttp
{
    { Route.Get("/users/{id}"), Reply.With(new User("octocat")) },
    { Route.Fallback(),         Reply.Status(HttpStatusCode.OK) },
};

[glennawatson]

Already implemented and shipped on main, so closing as completed.

• Route.Fallback() exists (src/Refit.Testing/Route.cs) and returns a matcher for any method/path.
• StubHttp now matches in three priority tiers (one-shot -> reusable -> fallback), so a fallback route is always tried last regardless of where it is declared in the table. This removes the old { Template = "*", Reusable = true }-as-last-entry workaround and its shadowing gotcha.
• Behaviour is covered by RouteTableFeatureTests.FallbackRouteIsTriedLastRegardlessOfDeclarationOrder, which declares the fallback first and asserts a specific route still wins, an unmatched request falls through to the fallback, and the fallback is not treated as a required expectation by VerifyAllCalledAsync.
• The new public surface (Route.Fallback(), RouteMatcher.Fallback) is in the shipped public API files.

Usage:

var http = new StubHttp
{
    { Route.Get("/users/{id}"), Reply.With(new User("octocat")) },
    { Route.Fallback(),         Reply.Status(HttpStatusCode.OK) },
};

[glennawatson]

This is addressed in the Refit 14 beta, now available on NuGet: https://www.nuget.org/packages/Refit/14.0.0-beta.1

Please give it a try - feedback ahead of the stable 14.0.0 release is very welcome.

Refit 14.0.0-beta.1

Type-safe REST API clients for .NET, generated from annotated interfaces with build-time source generation, direct request construction, System.Text.Json defaults, and support for trimming and AOT.