feat(ai): add Vercel AI Gateway routing support (#1051)

* feat(ai): add Vercel AI Gateway routing support

Add vercelGatewayRouting to OpenAICompletionsCompat, parallel to
openRouterRouting. When a model targets ai-gateway.vercel.sh and has
vercelGatewayRouting configured, the openai-completions provider passes
providerOptions.gateway with only/order in the request body.

Changes:
- types.ts: VercelGatewayRouting interface + field on OpenAICompletionsCompat
- openai-completions.ts: buildParams passes providerOptions.gateway,
  detectCompat/getCompat include the new field
- model-registry.ts: VercelGatewayRoutingSchema for models.json validation
- test: updated Required<OpenAICompletionsCompat> in test fixture

* docs(coding-agent): add vercelGatewayRouting to custom models documentation

packages/coding-agent/docs/models.md

@@ -165,6 +165,7 @@ For providers with partial OpenAI compatibility, use the `compat` field:
 | `supportsUsageInStreaming` | Supports `stream_options: { include_usage: true }` (default: `true`) |
 | `maxTokensField` | Use `max_completion_tokens` or `max_tokens` |
 | `openRouterRouting` | OpenRouter routing config passed to OpenRouter for model/provider selection |
+| `vercelGatewayRouting` | Vercel AI Gateway routing config for provider selection (`only`, `order`) |
 
 Example:
 
@@ -191,3 +192,34 @@ Example:
   }
 }
 ```
+
+Vercel AI Gateway example:
+
+```json
+{
+  "providers": {
+    "vercel-ai-gateway": {
+      "baseUrl": "https://ai-gateway.vercel.sh/v1",
+      "apiKey": "AI_GATEWAY_API_KEY",
+      "api": "openai-completions",
+      "models": [
+        {
+          "id": "moonshotai/kimi-k2.5",
+          "name": "Kimi K2.5 (Fireworks via Vercel)",
+          "reasoning": true,
+          "input": ["text", "image"],
+          "cost": { "input": 0.6, "output": 3, "cacheRead": 0, "cacheWrite": 0 },
+          "contextWindow": 262144,
+          "maxTokens": 262144,
+          "compat": {
+            "vercelGatewayRouting": {
+              "only": ["fireworks", "novita"],
+              "order": ["fireworks", "novita"]
+            }
+          }
+        }
+      ]
+    }
+  }
+}
+```