Fix bench after recent updates (#1331)

## Motivation

A lot of changes happened without much attention to the state of exo
bench.

## Changes

Use TaggedModel for BenchChatCompletion so it serialises properly.
Don't break after gpt oss tool call to preserve parity with the rest of
the codebase.

## Why It Works

<!-- Explain why your approach solves the problem -->

## Test Plan

### Manual Testing
<img width="2856" height="678" alt="image"
src="https://github.com/user-attachments/assets/2e18cf0d-c0f8-467c-9763-1a6a59c8a327"
/>

Also tested GPT OSS tool calling in OpenCode

tmp/config_examples/opencode.json

@@ -0,0 +1,18 @@
+{
+  "$schema": "https://opencode.ai/config.json",
+  "model": "exo/mlx-community/gpt-oss-120b-MXFP4-Q8",
+  "provider": {
+    "exo": {
+      "api": "http://localhost:52415/v1",
+      "models": {
+        "mlx-community/gpt-oss-120b-MXFP4-Q8": {
+          "name": "GPT OSS 120B",
+          "limit": {
+            "context": 32768,
+            "output": 8192
+          }
+        }
+      }
+    }
+  }
+}