msgspec: Mandatory attributes follow optional ones

[maringuu]

Steps to reproduce

1. Download the NVD CVE schema
2. Generate a msgpsec model:

datamodel-codegen \
    --input $schema_json \
    --input-file-type jsonschema \
    --output-model-type 'msgspec.Struct' \
    --snake-case-field \
    --output "."

3. (Ignore the circular imports Importing generated code results in a circular dependency #836)
4. Look at the class CveItem (and most other classes as well). It cannot be imported because a mandatory attribute follows an optional one, which is invalid in the generated __init__.

Expected behavior

Assuming #836 is fixed, the model should import correctly.

Workaround

Use kw_only=True

Setup

$ datamodel-codegen --version
0.25.5

$ python --version
Python 3.11.8