Add chat image upload support

docs/api/chat_client.md

@@ -58,6 +58,82 @@ print(result['choices'][0]['message']['content'])
 
 ---
 
+### upload_chat_image
+
+Upload an image to FastGPT's chat file storage and return a normalized file reference.
+
+```python
+image = client.upload_chat_image(
+    appId="app-123",
+    chatId="chat-123",
+    file_path="example.png"
+)
+
+response = client.create_chat_completion(
+    chatId="chat-123",
+    messages=[
+        {
+            "role": "user",
+            "content": [
+                {"type": "text", "text": "Describe this image"},
+                {"type": "image_url", "image_url": {"url": image["url"]}},
+            ],
+        }
+    ],
+)
+```
+
+Returned file reference:
+
+```python
+{
+    "type": "img",
+    "name": "example.png",
+    "key": "chat/...",
+    "url": "https://...",
+    "previewUrl": "https://..."
+}
+```
+
+---
+
+### upload_chat_file
+
+Upload a generic chat file. This is the lower-level helper used by `upload_chat_image`.
+
+```python
+file_ref = client.upload_chat_file(
+    appId="app-123",
+    chatId="chat-123",
+    file_path="document.pdf",
+    file_type="doc"
+)
+```
+
+---
+
+### presign_chat_file_post_url
+
+Request a presigned upload URL for a chat file. The SDK supports both FastGPT response shapes:
+
+- Presigned POST: `{ "url": "...", "fields": {...}, "maxSize": ... }`
+- Presigned PUT: `{ "url": "...", "headers": {...}, "key": "...", "previewUrl": "..." }`
+
+---
+
+### presign_chat_file_get_url
+
+Request a temporary preview/download URL by file `key`.
+
+```python
+preview_url = client.presign_chat_file_get_url(
+    appId="app-123",
+    key="chat/..."
+)
+```
+
+---
+
 ### get_chat_histories
 
 Get chat histories for an application.