rewrite async test mocking example to use stub() with auto-restore

[fibibot]

The Testing async functions section in examples/tutorials/testing.md
demonstrates how to mock fetch when testing async functions. The previous
example had two problems that bite anyone who copies it:

1. globalThis.fetch = async (url: string) => … fails type-checking on
   stable Deno — the real fetch accepts URL | RequestInfo, not just
   string. Running deno test against the existing snippet on Deno
   2.7.14 produced TS2322: Type '(url: string) => Promise<Response>' is not assignable to type '…(input: URL | RequestInfo, init?: …)…'.
2. The plain reassignment never restored the original fetch, so the
   mock leaked into every later test in the file. The page didn't mention
   this, so a reader following the pattern would silently break test
   isolation.

This PR rewrites that single section to use stub from
@std/testing/mock bound with
a using declaration. The stub matches fetch's real signature (the
file type-checks) and the disposer restores the original when each test
scope exits. A :::tip callout explains why the old shape was wrong so
readers learn the rule, not just the snippet. Filenames also moved from
dashes to underscores per runtime/contributing/style_guide.md.

Verified by running both tests plus a third "fetch is restored" probe
against denoland/deno:latest (2.7.14 — canary tag isn't published to
Docker Hub right now): Check async_test.ts passed and all 3 tests
passed in 5ms. The output block in the page is the real deno test
output from that run.

Closes bartlomieju/orchid-inbox#46

[fibibot]

@bartlomieju ready for review when you have a moment.