Preserve structured JsonElement tool arguments in GitHub Copilot agent events

[Copilot]

Motivation and Context

Nested tool arguments from Copilot tool execution events were being flattened into raw JSON strings. That breaks structured round-tripping through the AG-UI event pipeline by double-encoding object/array arguments.

Description

• Argument preservation

  • Keep JsonValueKind.Object and JsonValueKind.Array as cloned JsonElement values instead of converting them to GetRawText().
  • Treat JsonValueKind.Undefined as null to avoid emitting ambiguous payloads.

• Behavioral impact

  • Primitive values continue to map as before.
  • Structured tool arguments now serialize downstream as JSON objects/arrays rather than stringified JSON blobs.

• Coverage

  • Update the existing unit test to assert object and array arguments remain structured JsonElement values.

arguments[property.Name] = property.Value.ValueKind switch
{
    JsonValueKind.Object => property.Value.Clone(),
    JsonValueKind.Array => property.Value.Clone(),
    JsonValueKind.Undefined => null,
    _ => /* existing primitive handling */
};

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.