support perplexity.ai responses in org-ai--normalize-response

[lneely]

I was encountering a problem using the perplexity.ai service with org-ai. The requests and responses were working properly, but the response was not being written back to the buffer in the #begin_ai (...) #end_ai block.

This PR updates the org-ai--normalize-response function to properly read the response from the perplexity.ai API.

Fixes:

• Empty response from perplexity.ai #119

[rksm]

Thank you!