fix(ai-proxy): query params in override.endpoint not being sent to LLMs

[shreemaan-abhishek]

Description

Although the ai-proxy plugin officially supports only openai as an LLM provider, the plugin can be used to proxy requests to azure-openai service as mentioned in this blog: https://docs.api7.ai/apisix/how-to-guide/ai-gateway/proxy-azure-openai-requests, by entering the correct URL in override.endpoint attribute of ai-proxy.

However, the azure-openai deployment URL contains some query params that wasn't being carried into the request (HTTP request made by ai-proxy plugin) to the LLM endpoint. This PR ensures that query params in override.endpoint are appropriately used.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)