fix: text field in MultiContent chat message is required

[daulet]

Describe the change
Can't omit text field in in multi content chat message, see the example below.

Provide OpenAI documentation link
Try

curl "https://api.openai.com/v1/chat/completions" \
  -X POST \
  -H "Content-Type: application/json" \
  -H "Authorization: Bearer $OPENAI_API_KEY" \
  -d '{
         "messages": [
           {
             "role": "user",
             "content": [
               {
                 "type": "text"
               },
               {
                 "type": "image_url",
                 "image_url": {
                   "url": "https://shellsamurai.com/wp-content/uploads/2023/04/tux.jpg"
                 }
               }
             ]
           }
         ],
         "model": "gpt-4o",
         "temperature": 0,
         "max_tokens": 1024,
         "top_p": 1,
         "stream": false
       }'

You'll get:

{
  "error": {
    "message": "Invalid chat format. Expected 'text' field in text type content part to be a string.",
    "type": "invalid_request_error",
    "param": null,
    "code": null
  }
}

Describe your solution
Don't emit the Text field.

Tests
Successful request to OpenAI endpoint.

Additional context

Issue: #XXXX

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.01%. Comparing base (774fc9d) to head (f616ab6).
Report is 60 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #846      +/-   ##
==========================================
+ Coverage   98.46%   99.01%   +0.55%     
==========================================
  Files          24       26       +2     
  Lines        1364     1418      +54     
==========================================
+ Hits         1343     1404      +61     
+ Misses         15        8       -7     
  Partials        6        6              

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[sashabaranov]

Hey @daulet, thank you for the PR!

I'm curious of how the API would handle the messages of image_url type with non-ommited text?

For example

{
    "type": "image_url",
    "text": "",
    "image_url": {
        "url": "https://shellsamurai.com/wp-content/uploads/2023/04/tux.jpg"
    }
}

We might want to go with a custom MarshalJSON() in this case.