Issue #3586425: Resolve "ai_ckeditor: streamGenerateContent causes 400 Bad Request with Gemini 2.x models"

Description

Testing instructions

1. Follow the instructions here: https://project.pages.drupalcode.org/ai/1.3.x/contribute/testing/full_patch_test/#setup-ai-ckeditor
2. Try with a provider that allows streaming (OpenAI) - it should stream.
3. Try with a provider that doesn't allow streaming (Gemini) - it should sit and wait and come in one dump.

Checklist

• I have linked the related issue in the MR title or description
• I have performed a self-review of my own code
• I have written testing instructions and verified them locally

AI Compliance

Note

Check the one that best describes your usage, or leave all unchecked if AI was not significantly used.

• AI Assisted Code
  Mainly written by a human; AI used for autocomplete or partial generation under full human supervision.
• AI Generated Code
  Mainly generated by AI, reviewed and approved by a human before this MR was created.
• Vibe Coded
  Generated by AI and only functionally reviewed before this MR was created.

Closes #3586425