feat(request): add get_param_as_media() method

[MannXo]

Summary of Changes

Adds a new public helper: Request.get_param_as_media(name, media_type=None, required=False, store=None, default=None) :
1- Deserializes a single query-string parameter using the app's configured media handlers.
2- If media_type is omitted, falls back to the app's default_media_type.
3- Stores the parsed value in store when provided.
4- Raises HTTPInvalidParam on parse errors (or when no suitable handler exists for non-JSON media).

ASGI support:
1- falcon.asgi.Request.get_param_as_media delegates to the synchronous WSGI implementation (parameter values are already in memory).
2- get_param_as_json() now delegates to get_param_as_media(..., MEDIA_JSON) (thin wrapper).

Related Issues

closes: #2549

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a33301e) to head (641f528).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2556   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           64        64           
  Lines         7866      7875    +9     
  Branches      1078      1080    +2     
=========================================
+ Hits          7866      7875    +9     

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