fix: improve backend selection logic in WithBackend function

- Add handling for the "BackendUnspecified" case in the WithBackend function, falling through to existing logic

Signed-off-by: Bo-Yi Wu <[email protected]>

Author: appleboy

provider/gemini/options.go

@@ -122,6 +122,8 @@ func WithBackend(val string) Option {
 			c.backend = genai.BackendVertexAI
 		case "BackendGeminiAPI":
 			c.backend = genai.BackendGeminiAPI
+		case "BackendUnspecified":
+			fallthrough
 		default:
 			c.backend = genai.BackendGeminiAPI
 		}