Python: build_model_schema puts pydantic constraint objects into field descriptions (breaks tool payload JSON)

Description

KernelJsonSchemaBuilder.build_model_schema() puts FieldInfo.metadata[0] into the field's description when a pydantic v2 field has constraints. In pydantic v2, metadata[0] is a constraint object (Ge(ge=0.0)), not a description, so:

1. the real Field(..., description=...) is lost from the tool schema
2. the resulting schema dict contains a Ge object and fails JSON serialization with TypeError: Object of type Ge is not JSON serializable, breaking function-calling payloads for OpenAI-compatible connectors

Regression introduced by #6469.

Reproduction

from pydantic import BaseModel, Field
import json
from semantic_kernel.schema.kernel_json_schema_builder import KernelJsonSchemaBuilder

class Params(BaseModel):
    top_p: float = Field(default=0.9, ge=0.0, le=1.0, description="nucleus sampling")

json.dumps(KernelJsonSchemaBuilder.build_model_schema(Params)["properties"]["top_p"])
# TypeError: Object of type Ge is not JSON serializable

Expected behavior

Constraint-only metadata is ignored for descriptions; field_info.description is used when present, so the schema stays JSON-serializable.

Environment

semantic-kernel python main (ca40aa72), pydantic v2, Python 3.12