test: replace cast() with isinstance asserts in integration tests

[vdusek]

Sweep integration tests to replace type-only cast('T', result) calls with runtime assert isinstance(result, T) checks. Where the value is the typed model, the assert both narrows the type for ty and verifies the response shape at runtime instead of trusting it blindly.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.73%. Comparing base (9cf3794) to head (df91af9).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #787      +/-   ##
==========================================
+ Coverage   97.67%   97.73%   +0.06%     
==========================================
  Files          48       48              
  Lines        4864     4864              
==========================================
+ Hits         4751     4754       +3     
+ Misses        113      110       -3     

Flag	Coverage Δ
integration	93.66% <ø> (+0.04%)	⬆️
unit	86.20% <ø> (ø)

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