Python: Map NoneType to JSON Schema "null" instead of "object" in schema builder

[LeSingh1]

Motivation

KernelJsonSchemaBuilder omitted NoneType from TYPE_MAPPING, so get_json_schema(type(None)) — and any union of 3+ members that includes None (e.g. int | str | None) — emitted {"type": "object"} for the None member instead of the correct {"type": "null"}. These schemas are sent to models during function calling / structured output, where an object type for a None member is misleading and can distort tool arguments.

The two-member Optional[T] path builds nullability separately and was already correct; this only affected NoneType reached directly or as one member of a 3+-member union.

Fix

Add NoneType (and the "None" / "NoneType" string names used by build_from_type_name) to TYPE_MAPPING, so None maps to JSON Schema {"type": "null"}.

Files

• python/semantic_kernel/schema/kernel_json_schema_builder.py
• python/tests/unit/schema/test_schema_builder.py — regression tests

Verification

Added regression tests covering type(None) directly and a 3-member union containing None; they fail before the change ({"type":"object"}) and pass after ({"type":"null"}). Existing Optional[T] behavior is unchanged.

[Copilot]

Pull request overview

This PR fixes Python JSON Schema generation for NoneType in KernelJsonSchemaBuilder, ensuring None is represented as JSON Schema {"type": "null"} (instead of defaulting to {"type": "object"}) when encountered directly or as a member of larger unions used for function calling / structured output.

Changes:

• Add type(None) to TYPE_MAPPING so get_json_schema(type(None)) and union-member schema generation emit "null".
• Add "None" and "NoneType" string mappings so build_from_type_name(...) correctly emits "null".
• Add regression tests covering type(None), build(type(None)), Union[int, str, None], and "None"/"NoneType" type-name paths.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
python/semantic_kernel/schema/kernel_json_schema_builder.py	Maps type(None) / "None" / "NoneType" to JSON Schema "null" via TYPE_MAPPING.
python/tests/unit/schema/test_schema_builder.py	Adds regression tests for direct NoneType and 3+ member unions containing None.

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