fix(OpenAPI): Ensure default values are always represented in schema for dataclasses and msgspec.Structs

[provinzkraut]

Fix a bug that would prevent default values for dataclasses and msgspec.Struct`s to be included in the OpenAPI schema.

@dataclass
class SomeModel:
    field_a: str = "default_a"
    field_b: str = dataclasses.field(default="default_b")

and

class SomeModel(msgspec.Struct, kw_only=True):
    field_a: str = "default_a"
    field_b: str = msgspec.field(default="default_b")

now generate correct schemas in all cases.

Fix #3201.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.23%. Comparing base (62f2cee) to head (074d73f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3285   +/-   ##
=======================================
  Coverage   98.23%   98.23%           
=======================================
  Files         320      320           
  Lines       14451    14454    +3     
  Branches     2297     2297           
=======================================
+ Hits        14196    14199    +3     
  Misses        114      114           
  Partials      141      141           

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[sonarcloud]

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 SonarCloud

[github-actions]

Documentation preview will be available shortly at https://litestar-org.github.io/litestar-docs-preview/3285