Refa: make RAGFlow more asynchronous (#11601)

### What problem does this PR solve?

Try to make this more asynchronous. Verified in chat and agent
scenarios, reducing blocking behavior. #11551, #11579.

However, the impact of these changes still requires further
investigation to ensure everything works as expected.

### Type of change

- [x] Refactoring

api/utils/api_utils.py

@@ -22,6 +22,7 @@ import os
 import time
 from copy import deepcopy
 from functools import wraps
+from typing import Any
 
 import requests
 import trio
@@ -45,11 +46,40 @@ from common import settings
 requests.models.complexjson.dumps = functools.partial(json.dumps, cls=CustomJSONEncoder)
 
 
-async def request_json():
+async def _coerce_request_data() -> dict:
+    """Fetch JSON body with sane defaults; fallback to form data."""
+    payload: Any = None
+    last_error: Exception | None = None
+
     try:
-        return await request.json
-    except Exception:
-        return {}
+        payload = await request.get_json(force=True, silent=True)
+    except Exception as e:
+        last_error = e
+        payload = None
+
+    if payload is None:
+        try:
+            form = await request.form
+            payload = form.to_dict()
+        except Exception as e:
+            last_error = e
+            payload = None
+
+    if payload is None:
+        if last_error is not None:
+            raise last_error
+        raise ValueError("No JSON body or form data found in request.")
+
+    if isinstance(payload, dict):
+        return payload or {}
+
+    if isinstance(payload, str):
+        raise AttributeError("'str' object has no attribute 'get'")
+
+    raise TypeError(f"Unsupported request payload type: {type(payload)!r}")
+
+async def get_request_json():
+    return await _coerce_request_data()
 
 def serialize_for_json(obj):
     """
@@ -137,7 +167,7 @@ def validate_request(*args, **kwargs):
     def wrapper(func):
         @wraps(func)
         async def decorated_function(*_args, **_kwargs):
-            errs = process_args(await request.json or (await request.form).to_dict())
+            errs = process_args(await _coerce_request_data())
             if errs:
                 return get_json_result(code=RetCode.ARGUMENT_ERROR, message=errs)
             if inspect.iscoroutinefunction(func):
@@ -152,7 +182,7 @@ def validate_request(*args, **kwargs):
 def not_allowed_parameters(*params):
     def decorator(func):
         async def wrapper(*args, **kwargs):
-            input_arguments = await request.json or (await request.form).to_dict()
+            input_arguments = await _coerce_request_data()
             for param in params:
                 if param in input_arguments:
                     return get_json_result(code=RetCode.ARGUMENT_ERROR, message=f"Parameter {param} isn't allowed")