.NET: Bug: FoundryEvals sends `azure_ai_evaluator` testing-criteria type — HTTP 400 from current Foundry endpoints

@alliscode is already working on this.

Since Jul 8, 2026.

## Summary

`FoundryEvals.EvaluateAsync` always returns **HTTP 400** against current Azure AI Foundry project endpoints (`*.services.ai.azure.com/api/projects/*`). The service rejects the request body with:

```
HTTP 400 (invalid_request_error)
Invalid set of testing criteria. Invalid value: 'azure_ai_evaluator'.
Supported values are: [label_model, text_similarity, string_check, label_model, score_model, python, endpoint]
```

## Root Cause

In `dotnet/src/Microsoft.Agents.AI.Foundry/Evaluation/FoundryEvalWireModels.cs`, the `WireTestingCriterion` type defaults to `"azure_ai_evaluator"`:

```csharp
internal sealed class WireTestingCriterion
{
[JsonPropertyName("type")]
public string Type { get; init; } = "azure_ai_evaluator"; // ← rejected by current API
...
}
```

The current Foundry Evals REST API at `*.services.ai.azure.com` no longer accepts `azure_ai_evaluator` as a criterion type. The bug is present in the current `main` branch and in the published nightly `1.12.0-preview.260629.1`.

## Repro

```csharp
// Microsoft.Agents.AI.Foundry 1.12.0-preview.260629.1
// Azure.AI.Projects 2.1.0-beta.3
// Endpoint: https://.services.ai.azure.com/api/projects/

AIProjectClient client = new(new Uri(endpoint), new DefaultAzureCredential());
FoundryEvals evals = new(client, "gpt-4o", FoundryEvals.Relevance, FoundryEvals.TaskAdherence);

// Throws ClientResultException: HTTP 400 (invalid_request_error)
await agent.EvaluateAsync(queries, evals);
```

The Azure AD authentication succeeds. The failure is in the POST to `CreateEvaluationAsync` where the request body contains `"type": "azure_ai_evaluator"` in `testing_criteria`.

## Suggested Fix

Change the default or routing logic so built-in named evaluators use the supported criteria type (likely `"score_model"` with appropriate prompt configuration, or a new `azure_ai_evaluator`-equivalent type once the API exposes it). All three evaluation paths (`EvaluateAsync`, `EvaluateTracesAsync`, `EvaluateFoundryTargetAsync`) call `FoundryEvalConverter.BuildTestingCriteria` which produces `WireTestingCriterion` instances — fixing the wire model type fixes all paths.

## Environment

| Item | Value |
|---|---|
| Package | `Microsoft.Agents.AI.Foundry 1.12.0-preview.260629.1` |
| `Azure.AI.Projects` | `2.1.0-beta.3` |
| Foundry endpoint format | `https://.services.ai.azure.com/api/projects/` |
| .NET | `net10.0` |
| Auth | `DefaultAzureCredential` (working, HTTP 400 is not an auth error) |

## Workaround

Wrap `FoundryEvals` in a try/catch and degrade gracefully to local AgentEval metrics. See [AgentEval samples 12 & 13](https://github.com/AgentEvalHQ/AgentEval) for a reference implementation of this pattern.