docs: clarify router-agnostic middleware runs after route resolution

[stareezy-1]

Summary

Fixes #933

Problem

The UseMiddleware doc comment claims the middleware stack executes before route matching:

The middleware stack for any API will execute before searching for a matching route to a specific handler, which provides opportunity to respond early...

This is not what happens. Router-agnostic middleware runs inside the matched operation handler chain (api.Middlewares().Handler(...)), so the adapter router resolves the route first. An unmatched request is answered by the router with a 404 and never enters the middleware chain:

GET /unmatched  →  404, middleware not called
GET /matched    →  middleware chain runs, then the handler

Why docs and not a behavior change: middleware receives a huma.Context bound to the matched operation (ctx.Operation()), which cannot exist before route resolution. Moving the chain before routing would require a context redesign across all adapters. Router-specific middleware is the supported way to run pre-routing logic (see the middleware docs).

Fix

1. Correct the UseMiddleware doc comment to describe the actual behavior: the stack runs after the adapter router matches the request to a registered operation, and unmatched requests do not pass through it.
2. Add a regression test locking in the behavior.

Tests

TestMiddlewareDoesNotRunForUnmatchedRoute added to huma_test.go:

Case	Verifies
GET /unmatched	router answers 404, middleware flag stays false
GET /matched	middleware runs and the handler executes

All existing tests continue to pass (go test -race ./...).