♻️(engine): extract custom header parsing and update OpenAiEngine

- export parseCustomHeaders from src/utils/engine.ts
- use parseCustomHeaders in OpenAiEngine for config.customHeaders
- remove try/catch and inline JSON.parse logic
- update config test to expect headers as object and drop JSON.parse

Centralize header parsing for reuse and simplify engine code
Update tests to match new header format for clarity

src/utils/engine.ts

@@ -12,7 +12,7 @@ import { GroqEngine } from '../engine/groq';
 import { MLXEngine } from '../engine/mlx';
 import { DeepseekEngine } from '../engine/deepseek';
 
-function parseCustomHeaders(headers: any): Record<string, string> {
+export function parseCustomHeaders(headers: any): Record<string, string> {
   let parsedHeaders = {};
   
   if (!headers) {