fix(python): preserve Pydantic field descriptions in schemas

[S.J. (cstolting-collab)]

Motivation and Context

Description

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[S.J. (cstolting-collab)]

Summary

• Prefer Pydantic FieldInfo.description when building model field schemas.
• Preserve the existing metadata-dictionary fallback for compatibility.
• Add a regression test for constrained Pydantic fields.

Fixes #14443

Validation

Before this change, a constrained Pydantic field placed Ge(ge=0.0) in the generated schema description, causing json.dumps() to raise TypeError.

Verified after the change:

• uv run pytest tests/unit/schema/test_schema_builder.py — 37 passed
• `uv run pre-commit run --files semantic_kernel/schema/kernel_json_schema_builder.py tests/unit/schema/tes

[Copilot]

🟢 Approval recommended

The change is small, targeted, and includes a regression test validating the intended behavior for constrained Pydantic fields.

Pull request overview

This PR updates the Python JSON schema builder to correctly preserve Pydantic field description values when the field also has constraints (e.g., ge/le), and adds a unit test to prevent regressions. This fits into the semantic_kernel schema generation path used to produce structured output schemas from Python/Pydantic models.

Changes:

• Prefer field_info.description when deriving per-field schema descriptions from Pydantic model_fields.
• Avoid incorrectly treating Pydantic constraint metadata as a field description.
• Add a unit test covering a constrained Pydantic field with a description.

File summaries

File	Description
python/semantic_kernel/schema/kernel_json_schema_builder.py	Adjusts how field descriptions are extracted from Pydantic model_fields to preserve description even when constraints exist.
python/tests/unit/schema/test_schema_builder.py	Adds coverage to ensure constrained Pydantic fields keep the intended description in generated schemas.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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