feat(ai): add OpenRouter provider routing support

Allows custom models to specify which upstream providers OpenRouter
should route requests to via the `openRouterRouting` field in model
definitions.

Supported fields:
- `only`: list of provider slugs to exclusively use
- `order`: list of provider slugs to try in order

packages/ai/test/openai-completions-tool-result-images.test.ts

@@ -30,6 +30,7 @@ const compat: Required<OpenAICompletionsCompat> = {
 	requiresThinkingAsText: false,
 	requiresMistralToolIds: false,
 	thinkingFormat: "openai",
+	openRouterRouting: {},
 };
 
 function buildToolResult(toolCallId: string, timestamp: number): ToolResultMessage {