Add Generic for TestClient.app

[Dreamsorcerer]

This should avoid annoying assert client.app is not None lines in test code.

Fixes #7226.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.28%. Comparing base (f569894) to head (d1d88a2).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8977   +/-   ##
=======================================
  Coverage   98.28%   98.28%           
=======================================
  Files         107      107           
  Lines       34287    34295    +8     
  Branches     4062     4065    +3     
=======================================
+ Hits        33699    33707    +8     
  Misses        415      415           
  Partials      173      173           

Flag	Coverage Δ
CI-GHA	98.17% <83.33%> (-0.01%)	⬇️
OS-Linux	97.83% <83.33%> (-0.01%)	⬇️
OS-Windows	96.24% <83.33%> (-0.01%)	⬇️
OS-macOS	97.51% <83.33%> (-0.01%)	⬇️
Py-3.10.11	97.59% <66.66%> (-0.02%)	⬇️
Py-3.10.14	97.52% <66.66%> (-0.02%)	⬇️
Py-3.11.9	97.76% <83.33%> (-0.01%)	⬇️
Py-3.12.5	97.88% <83.33%> (-0.01%)	⬇️
Py-3.9.13	97.48% <66.66%> (-0.02%)	⬇️
Py-3.9.19	97.42% <66.66%> (-0.02%)	⬇️
Py-pypy7.3.16	97.03% <66.66%> (-0.02%)	⬇️
VM-macos	97.51% <83.33%> (-0.01%)	⬇️
VM-ubuntu	97.83% <83.33%> (-0.01%)	⬇️
VM-windows	96.24% <83.33%> (-0.01%)	⬇️

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