When using snake-case-fields and msgspec, field ordering does not always work correctly

[ianbuss]

Describe the bug
A clear and concise description of what the bug is.

To Reproduce

Example schema: https://raw.githubusercontent.com/kubernetes/kubernetes/release-1.28/api/openapi-spec/v3/api__v1_openapi.json

Used commandline:

$ datamodel-codegen --url 'https://raw.githubusercontent.com/kubernetes/kubernetes/release-1.28/api/openapi-spec/v3/api__v1_openapi.json' --output v1_28/ --output-model-type msgspec.Struct --snake-case-field --keep-model-order

Expected behavior
I expect field ordering to be required then optional fields. With snake-casing, the field ordering remains as it is in the source spec, which can result in import failures for generated models when optional fields appear before required fields.

Version:

• OS: macOS
• Python version: 3.11
• datamodel-code-generator version: 0.25.6

Additional context
n/a