Python: Exclude ClassVar annotations from generated JSON schemas

[Pushpak731]

Fixes #14527

Description

KernelJsonSchemaBuilder.build_model_schema builds the schema from get_type_hints, which includes ClassVar annotations, so a class constant became a required object-valued property in the generated schema. Pydantic excludes ClassVars from model_fields and from serialized instances, so the resulting schema rejected valid serialized payloads ({'value': 1} failed because marker was required). The same wrong schema reached KernelParameterMetadata, generated function-call tool definitions and structured_output=True schemas.

The fix skips ClassVar-annotated hints before the required/property loop — covering the parameterized form (ClassVar[str]), the bare form (ClassVar, whose get_origin is None), and inherited ClassVars, for KernelBaseModel subclasses, plain annotated classes and model instances alike.

class Payload(KernelBaseModel):
    marker: ClassVar[str] = "constant"
    value: int

KernelParameterMetadata(name="payload", type_object=Payload).schema_data
# before: properties include marker (type object) and required: [marker, value]
# after:  {'properties': {'value': ...}, 'required': ['value']}

Contribution Checklist

• I have read the contribution guidelines.
• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submit framework
• Tests are included and pass locally: tests/unit/schema/ (45 passed) and tests/unit/functions/test_kernel_parameter_metadata.py (7 passed) — 10 new regression tests cover pydantic models, bare/inherited ClassVar, plain classes, instances, structured_output, descriptions, optionals and the serialized-payload contract
• ruff check and ruff format --check are clean on both changed files
• Documentation updates are not needed (no public API change; behavior now matches model_dump)

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[Pushpak731]

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