Add OPENAI_BASE_URL support for custom OpenAI-compatible endpoints (#283)

Enables GPT models to use custom API endpoints through OPENAI_BASE_URL environment variable. This addresses the limitation where only Azure OpenAI supported custom endpoints while standard GPT models were hardcoded to api.openai.com.

- Add intelligent URL handling to avoid duplicate /v1 paths
- Maintain backward compatibility with default OpenAI API
- Update README with configuration instructions
- Non-breaking change preserving existing functionality

Fixes API integration issues for users with custom OpenAI-compatible services.

README.md

@@ -150,6 +150,11 @@ Set **OPENAI_API_KEY** environment variable with your API key
 export OPENAI_API_KEY='changeme'
 ```
 
+Optionally, set **OPENAI_BASE_URL** to use a custom OpenAI-compatible API endpoint
+```bash
+export OPENAI_BASE_URL='http://your-custom-endpoint.com/v1'  # Optional: defaults to https://api.openai.com
+```
+
 ```bash
 python run.py --path_to_vm Ubuntu/Ubuntu.vmx --headless --observation_type screenshot --model gpt-4-vision-preview --result_dir ./results
 ```