chore: pydanticv2 deprecation warning - access model_fields through class

[epmog]

What was the problem/requirement? (What/Why)

Warnings when running tests:

$ hatch run test
...
==== warnings summary ====
test/openjd/model/_internal/test_create_job.py: 35 warnings
test/openjd/model/test_create_job.py: 20 warnings
test/openjd/model/test_step_dependency_graph.py: 602 warnings
test/openjd/model/v2023_09/test_create.py: 49 warnings
test/openjd/model/test_step_param_space_iter.py: 10 warnings
 openjd-model-for-python/src/openjd/model/_internal/_create_job.py:83: PydanticDeprecatedSince211: Accessing the 'model_fields' attribute on the instance is deprecated. Instead, you should access this attribute from the model class. Deprecated in Pydantic V2.11 to be removed in V3.0.
    for field_name in model.model_fields.keys():
...
==== 2154 passed, 716 warnings in 4.18s ====

What was the solution? (How)

Use model.__class__ to access the values

What is the impact of this change?

No more warnings in this package and downstream consumers (ie. openjd-cli)!

How was this change tested?

$ hatch build && hatch run lint && hatch run test
...
==== 2154 passed in 4.29s ====

Was this change documented?

N/A

Is this a breaking change?

N/A

Does this change impact security?

N/A

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud