feat: support base64 and upload image input modes in chat

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>

fastgpt_client/__init__.py

@@ -17,6 +17,14 @@ from fastgpt_client.exceptions import (
     StreamParseError,
     ValidationError,
 )
+from fastgpt_client.images import (
+    decode_data_url,
+    encode_image_data_url,
+    image_part_from_bytes,
+    image_part_from_path,
+    image_url_part,
+    is_data_url,
+)
 from fastgpt_client.streaming import aiter_stream_events, iter_stream_events
 from fastgpt_client.stream_types import FastGPTInteractiveEvent, FastGPTStreamEvent
 
@@ -41,6 +49,13 @@ __all__ = [
     "aiter_stream_events",
     "FastGPTStreamEvent",
     "FastGPTInteractiveEvent",
+    # Image helpers
+    "encode_image_data_url",
+    "decode_data_url",
+    "is_data_url",
+    "image_url_part",
+    "image_part_from_bytes",
+    "image_part_from_path",
 ]
 
 __version__ = "0.1.0"