[Leon.C (zichen0116)]

Motivation and Context

Closes #14443.

In pydantic v2, FieldInfo.metadata is a list of constraint/annotation objects (e.g. [Ge(ge=0.0), Le(le=1.0)]), not the field description. KernelJsonSchemaBuilder.build_model_schema() took metadata[0] verbatim as the description, which caused two problems:

1. The real Field(..., description="...") was silently dropped from the generated tool schema.
2. The schema dict ended up containing a Ge/Le object, so json.dumps(schema) raised TypeError: Object of type Ge is not JSON serializable — breaking function-calling payloads for OpenAI-compatible connectors.

Description

In python/semantic_kernel/schema/kernel_json_schema_builder.py:

• The list-metadata branch now only treats a plain str entry as a description.
• Otherwise it falls back to field_info.description (where pydantic v2 actually stores it).
• The dict-metadata branch is unchanged.

Validation

• Reproduced the issue on current main before the fix: description came out as Ge(ge=0.0) and json.dumps raised TypeError.
• Added two regression tests in python/tests/unit/schema/test_schema_builder.py:
  • test_build_model_schema_with_constrained_field_with_description — constrained field with description="nucleus sampling" keeps the string description and the schema serializes.
  • test_build_model_schema_with_constrained_field_without_description — constrained field without a description yields no description key and the schema serializes.
• Both new tests fail on the unfixed code and pass with the fix.
• pytest tests/unit/schema/test_schema_builder.py: 38 passed (36 existing + 2 new).
• ruff check and ruff format --check clean on both touched files.

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