Typing fixes in file_handler: avoid cast()

[ogenstad]

Why

Improve typing issues in file_handler and remove ty violations.

What changed

• The potentially weird part to highlight here is the new function _open_binary() everything should work as before the problem was that the return types from anyio hid the actual type.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@                Coverage Diff                @@
##           infrahub-develop     #996   +/-   ##
=================================================
  Coverage             81.43%   81.43%           
=================================================
  Files                   134      134           
  Lines                 11359    11362    +3     
  Branches               1703     1703           
=================================================
+ Hits                   9250     9253    +3     
  Misses                 1566     1566           
  Partials                543      543           

Flag	Coverage Δ
integration-tests	41.90% <37.50%> (-0.01%)	⬇️
python-3.10	54.39% <62.50%> (-0.03%)	⬇️
python-3.11	54.40% <62.50%> (-0.01%)	⬇️
python-3.12	54.40% <62.50%> (+0.01%)	⬆️
python-3.13	54.40% <62.50%> (-0.01%)	⬇️
python-3.14	54.40% <62.50%> (+0.01%)	⬆️
python-filler-3.12	22.73% <37.50%> (+0.01%)	⬆️

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Files with missing lines	Coverage Δ
infrahub_sdk/file_handler.py	82.06% <100.00%> (+0.37%)	⬆️

... and 1 file with indirect coverage changes

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