docs: fix middleware testing example

[Yusufihsangorgel]

docs: fix broken middleware testing example

What was broken

The middleware test on the Testing
page does not compile-and-pass as written. Running it throws:

type 'Null' is not a subtype of type 'RequestContext'
  test/routes/_middleware_test.dart 7:7     _MockRequestContext.provide
  package:dart_frog/src/provider.dart 8:41  provider.<fn>.<fn>
  test/routes/_middleware_test.dart 25:20   main.<fn>.<fn>

The provider middleware injects its value by calling context.provide(...):

// packages/dart_frog/lib/src/provider.dart
return (context) => handler(context.provide(() => create(context)));

RequestContext.provide returns a (non-nullable) RequestContext. The example
only stubbed context.request, never context.provide, so on the mock context
provide returned null. That null is then passed straight into the wrapped
handler, which expects a RequestContext, producing the type error above. The
old example's context.read<String>() line is never reached.

The fix

Stub context.provide<String> to return the mocked context, then capture the
callback the provider handed it and assert that invoking it yields the expected
value. This is the same pattern the repo's own examples already use
(examples/kitchen_sink/test/routes/_middleware_test.dart,
examples/counter/test/routes/_middleware_test.dart), so the docs now match the
maintained code.

// Arrange
final handler = middleware((_) => Response(body: ''));
final request = Request.get(Uri.parse('http://localhost/'));
final context = _MockRequestContext();
when(() => context.request).thenReturn(request);
when(() => context.provide<String>(any())).thenReturn(context);

// Act
await handler(context);

// Assert
final create = verify(() => context.provide<String>(captureAny()))
    .captured
    .single as String Function();
expect(create(), equals('Hello World'));

The surrounding prose and the :::note were updated to explain the
context.provide stub, which is the crux of the fix.

Evidence

Reproduced against packages/dart_frog (v1.2.6) with a scratch project using the
verbatim documented example.

Before (documented example, verbatim):

00:00 +0 -1: middleware provides greeting [E]
  type 'Null' is not a subtype of type 'RequestContext'
  test/routes/_middleware_test.dart 7:7     _MockRequestContext.provide
  package:dart_frog/src/provider.dart 8:41  provider.<fn>.<fn>
00:00 +0 -1: Some tests failed.

After (corrected example):

00:00 +1: All tests passed!

The corrected test is not a tautology: changing the middleware to provide a
different value (e.g. 'Goodbye') makes it fail with
Expected: 'Hello World' Actual: 'Goodbye', confirming it exercises the
middleware's provided value rather than a hard-coded stub.

This fixes the broken example reported in #1963.

Note: #1963 also raises a broader request (a public RequestContext constructor / mock-free testing). This PR only corrects the documented example and intentionally does not close that discussion — so #1963 can stay open for the API question.