Fix discriminator mapping resolution in schemas with parent-child hierarchy

[sternakt]

This PR addresses #1832, where using discriminators in schemas with specific property mappings can cause a RuntimeError: Discriminator type is not found.

Background

The error occurs because the current discriminator mapping resolution fails to properly handle cases where the discriminator field appears in parent models, preventing the correct mapping key from being located.

Problem Description

Previously, the code only checked the discriminator_model path against the mapping keys, but it did not account for scenarios where:

The discriminator path does not directly match the path of the child models, although it matches their parent classes.
Paths need to be validated in a more recursive fashion, checking both parent and child models for a valid mapping.

Solution

The proposed solution introduces a helper function, check_paths(), to validate the paths for both the main discriminator_model and its base classes when the initial mapping fails. This function:

• Verifies mapping keys in both the main model and its parent classes.

Summary

• Introduced check_paths() function to handle discriminator mapping resolution.
• Ensured that the discriminator field is validated against both the main model and its parent models, preventing RuntimeError.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (cb0462a) to head (550b5ab).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
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  Coverage   100.00%   100.00%           
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  Files           38        38           
  Lines         4259      4263    +4     
  Branches       984       984           
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+ Hits          4259      4263    +4     

Flag	Coverage Δ
unittests	99.67% <100.00%> (+<0.01%)	⬆️

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[codspeed-hq]

CodSpeed Performance Report

Merging #2145 will not alter performance

_{Comparing sternakt:main (550b5ab) with main (cb0462a)}

Summary

✅ 30 untouched benchmarks

[koxudaxi]

Thank you very much!!