Fix: Allow OCO_API_URL to override DeepSeek engine baseURL

- Move hardcoded baseURL before ...config spread in constructor
- This allows user config to override the default DeepSeek API URL
- Fixes issue #539 where OCO_API_URL was ignored by DeepSeek engine

Author: gaozhenqian

src/engine/deepseek.ts

@@ -10,9 +10,10 @@ export interface DeepseekConfig extends OpenAiConfig {}
 export class DeepseekEngine extends OpenAiEngine {
   constructor(config: DeepseekConfig) {
     // Call OpenAIEngine constructor with forced Deepseek baseURL
+    // Put baseURL first so user config can override it
     super({
-      ...config,
-      baseURL: 'https://api.deepseek.com/v1'
+      baseURL: 'https://api.deepseek.com/v1',
+      ...config
     });
   }