test: do not use deprecated and removed APIs of httpx

[WSH032]

• proxies => mounts
• app => transport

Summary

Checklist

• I've read CONTRIBUTING.md.
• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.96%. Comparing base (977d9c1) to head (3622e3a).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
+ Coverage   96.74%   96.96%   +0.21%     
==========================================
  Files           9        9              
  Lines         461      461              
  Branches       67       44      -23     
==========================================
+ Hits          446      447       +1     
  Misses          9        9              
+ Partials        6        5       -1     

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