[Platform][Gemini] Response with multiple content parts ends up with an error

[chr-hertel]

While working on #1054 I ran into the issue that those image models might generate a response with two content parts: 1 text part and one inlineData part - which is not handled well currently and makes that added example flaky - especially with gemini-2.5-flash-image.

Use the example gemini/image-editing.php for reproduction and this method is to fix:
Symfony\AI\Platform\Bridge\Gemini\Gemini\ResultConverter::convertChoice, see TODO comment

[NicolasGraph]

Thanks @chr-hertel for introducing gemini-2.5-flash-image and gemini-3-pro-image-preview models!
I can't see a way to return multiparts responses without returning multiple appropriate results

[microHoffman]

I got this issue even when I received multiple content parts with only text:

      "content": {
        "parts": [
          {
            "text": "\nThe Content...."
          },
          {
            "text": "Another content..."
          },
          {
            "text": "```json\n{\n  ... \n```"
          }
        ],
        "role": "model"
      },

Is there a reason why in this case we just do not concatenate the results? or e.g. return array?

[NicolasGraph]

@microHoffman, concatenating the result as proposed by @TNAJanssen in #1046 is not enough because multiple content parts can be of different types, as decribed by @chr-hertel. I think it is also not as simple as returning an array because of mutliple chained methods, but something like a ResultBag should maybe be introduced