feat (ai): support model-generated files

[lgrammel]

Fixes #5181

Tasks

• implementation
  • opt-in on google provider
  • google provider generate
  • google provider stream
  • pass image output through generateText (new property)
  • pass image output through streamTest (new parts)
  • assistant message image parts conversion core
  • assistant message image part conversion google
  • response messages generate
  • response messages stream
  • change assistant output to file parts
  • generated images vs generated files
  • step result include files
  • data stream file parts
  • ui message assistant file parts
  • converting ui messages to core messages
  • detect prompt type
  • replace GeneratedImage, add mimetypes
  • stream text result files
  • append msg
• testing
  • google model generate
  • google model stream
  • google model provider option
  • generate text returns
  • stream text returns
  • ui file part conversion
  • detect prompt type
  • convert to core msgs
  • single message persistence
• examples
  • generate text image output
  • generate text image chatbot
  • stream text image output
  • stream text image chatbot
  • ui chatbot with images
  • single message persistence
• documentation
  • prompt guide
  • google provider guide (option, image output) - model specific
  • add to image generation section
  • data stream file parts
  • ui chatbot guide
  • generateImage reference mimetype info
  • streamText reference
  • generateText reference
• changeset