fix(config): accept object-format fallback_models in Zod validation (fixes #2943)

FallbackModelsSchema now explicitly accepts string[], object[], and
mixed array formats, matching the JSON schema definition.

Verified: 15 config schema tests pass, typecheck clean

src/config/schema/fallback-models.ts

@@ -17,9 +17,15 @@ export const FallbackModelObjectSchema = z.object({
 
 export type FallbackModelObject = z.infer<typeof FallbackModelObjectSchema>
 
+export const FallbackModelStringArraySchema = z.array(z.string())
+export const FallbackModelObjectArraySchema = z.array(FallbackModelObjectSchema)
+export const FallbackModelMixedArraySchema = z.array(z.union([z.string(), FallbackModelObjectSchema]))
+
 export const FallbackModelsSchema = z.union([
   z.string(),
-  z.array(z.union([z.string(), FallbackModelObjectSchema])),
+  FallbackModelStringArraySchema,
+  FallbackModelObjectArraySchema,
+  FallbackModelMixedArraySchema,
 ])
 
 export type FallbackModels = z.infer<typeof FallbackModelsSchema>