refactor(pytest_plugin): migrate session state to pytest.Stash

[ogenstad]

Why

Found when analyzing the codebase for areas where we could improve typing. Use a new pytest.Stash feature to allow for correct typing within our pytest plugin

What changed

• Migrate the pytest plugin's session state from dynamic attribute assignment to pytest.StashKey

[codecov]

Codecov Report

❌ Patch coverage is 79.31034% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
infrahub_sdk/pytest_plugin/items/graphql_query.py	33.33%	2 Missing ⚠️
...frahub_sdk/pytest_plugin/items/jinja2_transform.py	33.33%	2 Missing ⚠️
infrahub_sdk/pytest_plugin/items/check.py	50.00%	1 Missing ⚠️
...frahub_sdk/pytest_plugin/items/python_transform.py	66.66%	1 Missing ⚠️

@@                 Coverage Diff                  @@
##           infrahub-develop     #985      +/-   ##
====================================================
+ Coverage             81.43%   81.45%   +0.01%     
====================================================
  Files                   134      135       +1     
  Lines                 11359    11371      +12     
  Branches               1703     1702       -1     
====================================================
+ Hits                   9250     9262      +12     
  Misses                 1566     1566              
  Partials                543      543              

Flag	Coverage Δ
integration-tests	41.86% <10.34%> (-0.05%)	⬇️
python-3.10	54.33% <31.03%> (-0.07%)	⬇️
python-3.11	54.34% <31.03%> (-0.07%)	⬇️
python-3.12	54.33% <31.03%> (-0.09%)	⬇️
python-3.13	54.33% <31.03%> (-0.09%)	⬇️
python-3.14	54.32% <31.03%> (-0.07%)	⬇️
python-filler-3.12	22.80% <79.31%> (+0.08%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
infrahub_sdk/pytest_plugin/_stash.py	100.00% <100.00%> (ø)
infrahub_sdk/pytest_plugin/items/base.py	78.37% <100.00%> (ø)
infrahub_sdk/pytest_plugin/loader.py	79.59% <100.00%> (+0.86%)	⬆️
infrahub_sdk/pytest_plugin/plugin.py	84.44% <100.00%> (-0.34%)	⬇️
infrahub_sdk/pytest_plugin/items/check.py	44.68% <50.00%> (+1.20%)	⬆️
...frahub_sdk/pytest_plugin/items/python_transform.py	61.11% <66.66%> (+0.73%)	⬆️
infrahub_sdk/pytest_plugin/items/graphql_query.py	50.00% <33.33%> (+1.51%)	⬆️
...frahub_sdk/pytest_plugin/items/jinja2_transform.py	60.86% <33.33%> (+0.57%)	⬆️

... and 1 file with indirect coverage changes

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