Use .net 9 json schema exporter for tool function schema generation

[StephenHodgson]

When .net 8 EOL comes, the custom JsonSchema generator in this library will be deprecated and replaced by the native system.text.json schema exporter:

https://devblogs.microsoft.com/dotnet/system-text-json-in-dotnet-9/#json-schema-exporter

[StephenHodgson]

• Blocked by JsonSchemaExporter should use $defs keyword for recurring type schemas. dotnet/runtime#113698