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.

mm_agents/agent.py

@@ -615,13 +615,17 @@ class PromptAgent:
             else:
                 return response.json()['choices'][0]['message']['content']
         elif self.model.startswith("gpt"):
+            # Support custom OpenAI base URL via environment variable
+            base_url = os.environ.get('OPENAI_BASE_URL', 'https://api.openai.com')
+            # Smart handling: avoid duplicate /v1 if base_url already ends with /v1
+            api_url = f"{base_url}/chat/completions" if base_url.endswith('/v1') else f"{base_url}/v1/chat/completions"
             headers = {
                 "Content-Type": "application/json",
                 "Authorization": f"Bearer {os.environ['OPENAI_API_KEY']}"
             }
             logger.info("Generating content with GPT model: %s", self.model)
             response = requests.post(
-                "https://api.openai.com/v1/chat/completions",
+                api_url,
                 headers=headers,
                 json=payload
             )
@@ -631,7 +635,7 @@ class PromptAgent:
                     logger.error("Context length exceeded. Retrying with a smaller context.")
                     payload["messages"] = [payload["messages"][0]] + payload["messages"][-1:]
                     retry_response = requests.post(
-                        "https://api.openai.com/v1/chat/completions",
+                        api_url,
                         headers=headers,
                         json=payload
                     )