Add share and direct links for image responses

[rong3000]

Summary

• add return_share_url support for image and chat image flows
• resolve Grok share pages to real assets.grok.com image URLs through the public rest/app-chat/share_links/<publicId> JSON, without requiring Chrome/CDP at runtime
• prefer final generated images over preview chunks when collecting image outputs
• update the built-in /chat page to show image results plus share/direct links for image-generation prompts
• keep normal /chat streaming behavior unchanged for non-image prompts

Testing

• python3 -m py_compile app/api/v1/chat.py app/api/v1/image.py app/services/grok/services/chat.py app/services/grok/services/image.py app/services/grok/utils/process.py app/services/grok/utils/share_resolver.py app/services/reverse/app_chat_share.py tests/test_openai_usage.py tests/test_image_share.py tests/test_process_images.py
• node --check _public/static/function/js/chat.js
• manual API verification:
  • POST /v1/images/share/resolve returns share_image_url as https://assets.grok.com/...
  • POST /v1/images/generations returns share_url and share_image_url
  • POST /v1/chat/completions with grok-4.20-beta returns image content plus share_url and share_image_url

Notes

• runtime share-link resolution no longer depends on a local browser session
• the /chat page only switches to non-stream responses for image-generation scenarios, so ordinary chat keeps the existing streaming UX