[OPIK-853] handle gemini client errors

[idoberko2]

Details

Gemini client errors were not handled correctly. The purpose of this PR is fix the Gemini error handling method to respond with a human readable error.
Gemini API throws RuntimeExceptions with the error details as a JSON inside the message. This PR adds parsing to the error message in order to extract the meaningful part out of it.

Issues

OPIK-853

Testing

Tested locally:

❯ curl -v -X "POST" -H "Content-Type: application/json" --data '{"model":"gemini-1.5-flash","messages":[{"role":"user","content":"why are the sky blue?"}],"stream":true,"stream_options":{"include_usage":true},"temperature":0,"max_completion_tokens":1024,"top_p":1}' http://localhost:8080/v1/private/chat/completions

* Host localhost:8080 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
*   Trying [::1]:8080...
* Connected to localhost (::1) port 8080
> POST /v1/private/chat/completions HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/8.7.1
> Accept: */*
> Content-Type: application/json
> Content-Length: 200
>
* upload completely sent off: 200 bytes
< HTTP/1.1 200 OK
< Date: Sun, 26 Jan 2025 11:23:34 GMT
< Content-Type: text/event-stream
< Transfer-Encoding: chunked
<
{"code":429,"message":"Resource has been exhausted (e.g. check quota).","details":"RESOURCE_EXHAUSTED"}
* Connection #0 to host localhost left intact

[thiagohora]

Minor suggestion: It looks good!

[BorisTkachenko]

Left a few comments

[andrescrz]

Please address the important open comments.