Add temperature override option in config

- Add temperature field to Config struct (optional override)\n- Pass tempOverride to ConvertOpenAIRequest\n- Use override temperature if set, otherwise use client's temperature\n- Document option in config.yaml with example\n- Update README with temperature override documentation

config.yaml

@@ -1,6 +1,11 @@
 port: 8080
 upstream_url: "https://api.z.ai/api/anthropic"
 
+# Temperature override for all requests (optional)
+# If set, this temperature will be used instead of what clients request
+# Remove this line or set to null to use client-specified temperatures
+# temperature: 0.7
+
 models:
   - id: "glm-4.7"
     owned_by: "zhipu"