Sets model.additionalProperties for composed schemas

[spacether]

Sets model.additionalProperties for composed schemas
This allows v2 spec additionalProperties values to default to {} (AnyType schema) when omitted
No generator uses this data yet.

Tests updated for v2 and v3 additionalProperties checking
v2 specs, legacy behavior (disallowAdditionalPropertiesIfNotPresent = True, supportsAdditionalPropertiesWithComposedSchema = False)

• additionalProperties unset/false/true -> model.additionalProperties = null
• additionalProperties schema -> model.additionalProperties = schema

v2 specs, new behavior (disallowAdditionalPropertiesIfNotPresent = False, supportsAdditionalPropertiesWithComposedSchema=True)

• additionalProperties unset/false/true -> model.additionalProperties = {} AnyType schema
• additionalProperties schema -> model.additionalProperties = schema

This update will be used in the python generator in #8802

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