fix unsafe annotation copying in llm_router decorator

Prevent potential code injection via arbitrary type annotations in the llm_router decorator.

## Changes
- Removed blanket copying of all annotations from decorated functions via `func.__annotations__.copy()`
- Now only copies the return annotation when needed for validation
- Added validation to ensure the annotation is an actual type object, not a string forward reference

## Why
The previous code copied all annotations from user-provided decorated functions to the wrapper. String annotations (forward references) can be evaluated by `typing.get_type_hints()` in the function's globals/locals namespace, potentially executing arbitrary code.

The fix restricts annotation copying to only the return type (which is actually used for routing validation) and rejects string annotations that could be dangerous when evaluated. The wrapper function already has proper parameter annotations defined in its signature, so those don't need to be copied.

## Semgrep Finding Details
Annotations passed to `typing.get_type_hints` are evaluated in `globals` and `locals` namespaces. Make sure that no arbitrary value can be written as the annotation and passed to `typing.get_type_hints` function.

@18578539 requested Semgrep Assistant generate this pull request to fix [a finding](https://semgrep.dev/orgs/rootflo_ai/findings/683091385) from the detection rule [python.lang.security.audit.dangerous-annotations-usage.dangerous-annotations-usage](https://semgrep.dev/r/python.lang.security.audit.dangerous-annotations-usage.dangerous-annotations-usage).

Author: Semgrep Autofix

flo_ai/flo_ai/arium/llm_router.py

@@ -1038,12 +1038,14 @@ async def wrapper(
         ):
             return await router_instance.route(memory, execution_context)
 
-        # Preserve the original function's type annotations including return type
-        wrapper.__annotations__ = func.__annotations__.copy()
-
-        # Ensure the return annotation is properly set
+        # Only copy the return annotation if needed for validation
+        # Avoid copying arbitrary string annotations which could be evaluated
+        # by get_type_hints() and potentially execute code
         if 'return' in func.__annotations__:
-            wrapper.__annotations__['return'] = func.__annotations__['return']
+            return_annotation = func.__annotations__['return']
+            # Only copy if it's an actual type object, not a forward reference string
+            if not isinstance(return_annotation, str):
+                wrapper.__annotations__['return'] = return_annotation
 
         return wrapper