Fix TLS redirect doc example to use startup()

[ChihweiLHBird]

Replace manual app.signalize(), app.finalize(), and app.state.is_started = True with await app_server.startup() in the HTTP-to-HTTPS redirect documentation

The create_server docstring explicitly recommends using await app_server.startup() for app initialization. The manual signalize()/finalize() approach skips steps that _startup() performs — extension setup, route duplicate checking, uvloop conflict detection, and touchup optimizations. While the manual approach works for trivial cases, startup() is the correct API and safer to recommend in docs where users may adapt the pattern for more complex apps.

Resolves #2832 (remaining work after #2864)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.731%. Comparing base (785d77f) to head (84c4b53).

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #3155       +/-   ##
=============================================
- Coverage   87.793%   87.731%   -0.062%     
=============================================
  Files          105       105               
  Lines         8143      8143               
  Branches      1290      1290               
=============================================
- Hits          7149      7144        -5     
- Misses         687       692        +5     
  Partials       307       307               

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