Fix spurious </think> tags caused by empty string reasoning_content in streaming (#7025)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: ekzhu <320302+ekzhu@users.noreply.github.com>
Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>

Author: Copilot

python/packages/autogen-ext/src/autogen_ext/models/openai/_openai_client.py

@@ -947,8 +947,8 @@ async def create_stream(
                     is_reasoning = True
                 thought_deltas.append(reasoning_content)
                 yield reasoning_content
-            elif is_reasoning:
-                # Exit reasoning mode.
+            elif reasoning_content is None and is_reasoning:
+                # Exit reasoning mode only when reasoning_content is None (not when it's an empty string).
                 reasoning_content = "</think>"
                 thought_deltas.append(reasoning_content)
                 is_reasoning = False