[python] fix: additionalProperties:true schema inherits object instead of BaseModel (#20153)

[Shaun-3adesign]

Fixes #20153

Schemas that declare additionalProperties: true alongside named properties were generating broken Python FastAPI (and Connexion) output — an invalid import and the wrong base class:

# Before (broken)
from openapi_server.models.object import object   # invalid — module does not exist

class HttpRequest(object):                        # wrong — should be BaseModel
    ...

# After (fixed)
from pydantic import BaseModel

class HttpRequest(BaseModel):                     # correct base class
    additional_properties: Dict[str, Any] = {}    # correct additional-props field
    ...

Root cause: ModelUtils.isMapSchema() returns true for any schema whose additionalProperties is the boolean true, even when the schema also has named properties. This caused DefaultCodegen.updateModelForObject() to call addAdditionPropertiesToCodeGenModel() → addParentContainer() → addParentFromContainer(), setting model.parent = toInstantiationType(schema). For additionalProperties: true, getSchemaType(new Schema()) returns "object", so model.parent becomes Python's built-in type name. AbstractPythonCodegen.postProcessAllModels() then adds it to imports, producing the invalid from openapi_server.models.object import object.

PythonClientCodegen already had a correct override that avoided this, but PythonFastAPIServerCodegen and AbstractPythonConnexionServerCodegen did not.

Fix: Move the correct addAdditionPropertiesToCodeGenModel() override from PythonClientCodegen up to AbstractPythonCodegen. All Python generators now share the same implementation: only additionalPropertiesType is set (used for typing in templates), and model.parent is left null so the template correctly falls back to BaseModel. The now-redundant override in PythonClientCodegen is removed along with the two unused imports it left behind.

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---

Summary by cubic

Fixes #20153. Python FastAPI and Connexion now generate correct models for schemas with additionalProperties: true: classes inherit BaseModel and no invalid object import is emitted.

• Bug Fixes
  • Moved addAdditionPropertiesToCodeGenModel to AbstractPythonCodegen; only set additionalPropertiesType and leave model.parent unset to fall back to BaseModel.
  • Removed the duplicate override from PythonClientCodegen.
  • Added a regression test (issue_20153.yaml) verifying class HttpRequest(BaseModel), presence of additional_properties, and absence of from openapi_server.models.object import object.
  • Re-generated Python client petstore samples with a fresh build to restore UploadFileWithAdditionalPropertiesRequestObject exports and correct type hints across six Python client samples.

^{Written for commit a66eea0. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

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