fix: add missing __aiter__ method to TextGenerationStream (#196)

Without __aiter__, async for chunk in stream: raises
TypeError: 'TextGenerationStream' object is not an async iterable.
This makes async streaming completely broken when _is_async=True.

Author: amathxbt

src/opengradient/types.py

@@ -318,6 +318,12 @@ def __iter__(self):
         """Iterate over stream chunks."""
         return self
 
+    def __aiter__(self):
+        """Return async iterator (required for async for loops)."""
+        if not self._is_async:
+            raise TypeError("Use __iter__ for sync iterators")
+        return self
+
     def __next__(self) -> StreamChunk:
         """Get next stream chunk."""
         import json