[BUG] `Azure.Mcp.Tests` framework produces bad error for bad TestMode setting

[scbedd]

Right now, a .testsettings is by default populated with the value TestMode: "Live"

When you provide an improper value (EG not in [Live, Record, Playback]), the test fails with a standard JSON validation error:

System.Text.Json.JsonException : The JSON value could not be converted to Azure.Mcp.Tests.Helpers.TestMode. Path: $.TestMode | LineNumber: 10 | BytePositionInLine: 20.

It's obviously easy to fix, but it should just straight up tell you

your TestMode is wrong and needs to be one of 3 values.