Map JSON DataContent to Part.Data instead of Part.Raw

[SergeyMenshykh]

Description

AIContentExtensions.ToPart() currently maps every Microsoft.Extensions.AI.DataContent instance to Part.Raw, regardless of its media type.

The reverse conversion already maps Part.Data to JSON DataContent. Making the forward conversion media-type-aware provides symmetric round-tripping and avoids representing structured JSON as opaque base64-encoded bytes.

Example

var json = JsonSerializer.SerializeToUtf8Bytes(new { key = "value" });
var content = new DataContent(json, "application/json");

Part? part = content.ToPart();

Current behavior

part.Raw contains the JSON bytes and part.Data is unset.

Expected behavior

For application/json and media types with a +json structured syntax suffix, such as application/problem+json:

• Parse the content as JSON.
• Set Part.Data.
• Preserve the original media type.
• Surface a JsonException when content declared as JSON is invalid.

Non-JSON DataContent should continue to map to Part.Raw.