refactor: prevent apiKey field creation for official providers

Improved useApiKeyState hook to explicitly handle category parameter:
- Only create apiKey field for non-official providers in add mode
- Explicitly check category !== undefined to avoid unintended behavior
- Added comprehensive comments explaining the logic
- Updated dependency array to include category parameter

This ensures official provider configs remain clean without empty apiKey fields.

src/components/providers/forms/ProviderForm.tsx

@@ -121,6 +121,7 @@ export function ProviderForm({
     initialConfig: form.watch("settingsConfig"),
     onConfigChange: (config) => form.setValue("settingsConfig", config),
     selectedPresetId,
+    category,
   });
 
   // 使用 Base URL hook (仅 Claude 模式)