Fix parameter resolving falling back to assumptions resolver for optional Union types

[mauvilsa]

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• Did you read the contributing guideline?
• [n/a] Did you update the documentation? (readme and public docstrings)
• Did you write unit tests such that there is 100% coverage on related code? (required for bug fixes and new features)
• Did you verify that new and existing tests pass locally?
• Did you make sure that all changes preserve backward compatibility?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (2de15dd) to head (68455fd).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #498   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        21           
  Lines         6116      6116           
=========================================
  Hits          6116      6116           

Flag	Coverage Δ
py3.10	87.23% <100.00%> (ø)
py3.10_all	98.64% <100.00%> (ø)
py3.10_pydantic1	49.06% <100.00%> (ø)
py3.10_pydantic2	48.83% <100.00%> (ø)
py3.10_types	98.65% <100.00%> (ø)
py3.11	87.21% <100.00%> (ø)
py3.11_all	98.64% <100.00%> (ø)
py3.11_types	98.65% <100.00%> (ø)
py3.12	87.36% <100.00%> (ø)
py3.12_all	98.64% <100.00%> (ø)
py3.12_types	98.65% <100.00%> (ø)
py3.7	87.77% <100.00%> (ø)
py3.7_all	99.25% <100.00%> (ø)
py3.7_types	99.25% <100.00%> (ø)
py3.8	87.94% <100.00%> (ø)
py3.8_all	99.41% <100.00%> (ø)
py3.8_types	99.41% <100.00%> (ø)
py3.9	87.78% <100.00%> (ø)
py3.9_all	99.28% <100.00%> (ø)
py3.9_types	99.29% <100.00%> (ø)

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