refactor(shared): add normalizeFallbackModels utility function

Add shared utility to normalize fallback_models config values.

Handles both single string and array inputs consistently.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/shared/model-resolver.ts

@@ -7,6 +7,17 @@ export type ModelResolutionInput = {
 	systemDefault?: string
 }
 
+/**
+ * Normalizes fallback_models to an array.
+ * Handles single string or array input, returns undefined for falsy values.
+ */
+export function normalizeFallbackModels(
+	fallbackModels: string | string[] | undefined | null
+): string[] | undefined {
+	if (!fallbackModels) return undefined
+	return Array.isArray(fallbackModels) ? fallbackModels : [fallbackModels]
+}
+
 export type ModelSource =
 	| "override"
 	| "category-default"