Enhance ASGI scope typing and add HTTP event types

[MustafaNazir11]

Summary of Changes

Introduce TypedDict definitions for ASGI HTTP event messages.

• Add TypedDicts for HTTP request, response start, response body, and disconnect events
• Provide structured typing for ASGI event payloads while keeping existing aliases unchanged

This improves type clarity and lays the groundwork for stricter typing in the future without affecting runtime behavior.

Related Issues

Relates to #2628

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (e9e1d4b) to head (3a5404e).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2638   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           64        64           
  Lines         7976      8009   +33     
  Branches      1102      1102           
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+ Hits          7976      8009   +33     

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[vytas7]

Hi @MustafaNazir11, and thanks for this PR.

The main goal of these improved annotations was to get rid of typing ignores, however, you haven't removed any.
See also inline comments.

You've checked that you ran tox yourself, but that is obviously not true (see the failing CI).

Taking all these points into account, I believe it would be more efficient to restart this PR from scratch.

[vytas7]

This is the hottest code path in Falcon, we simply cannot afford cast(...) here.

[vytas7]

I see how these could belong to asgi_spec.py too, but let's colocate these aliases in falcon/_typing.py for now.