Assistant MCP Extension server fails to start when it uses JSON Schema union types

How to reproduce:

1. Download the attached “IN-150260-server” and take note of it’s location
2. Create and open a new Unity project
3. Ensure that the Assistant package is installed
4. Navigate to “Project Settings → AI → Assistant MCP Extensions”
5. Under “General Settings”, select “Enable MCP Tools”
6. Under “Server Configuration”, open the config file
7. Replace the contents with the attached “IN-150260-config” file
8. Ensure that the paths within the config file match the paths on your system
9. In the Project Settings window within the Editor, select “Refresh File and Servers” under “Server Configuration”
10. Under “Servers”, press “Inspect” next to “union-type-repro”
11. Observe the Server Status and the Server Message

Actual result: Server fails to start with “Cannot cast Newtonsoft.Json.Linq.JArray to Newtonsoft.Json.Linq.JToken.”
Expected result: Server starts and the tools are loaded successfully

Reproducible with: 2.8.0-pre.1 (6000.3.22f1), 2.18.0-pre.1 (6000.7.0a5)

Reproducible on: macOS 26.5.2 (M4 Max)
Not reproducible on: No other environments tested