Add a unified way to send images in chat: inline base64 data URLs
(passed through natively) or auto-upload via image_input_mode="upload",
which replaces inline data URLs with hosted URLs using upload_chat_image.
- New fastgpt_client/images.py with content-part / data-URL helpers
- image_input_mode + appId/outLinkAuthData params on create_chat_completion
(sync and async); upload failures fall back to inline base64
- Tests covering helpers, both modes, validation, and fallback
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
