fix: stop leaking reasoning_content to stream output

The streaming path in OpenAICompatProvider.chat_stream() was passing
reasoning_content deltas through on_content_delta(), causing model
internal reasoning to be displayed to the user alongside the actual
response content.

reasoning_content is already collected separately in _parse_chunks()
and stored in LLMResponse.reasoning_content for session history.
It should never be forwarded to the user-facing stream.

nanobot/providers/openai_compat_provider.py

@@ -671,9 +671,6 @@ class OpenAICompatProvider(LLMProvider):
                     break
                 chunks.append(chunk)
                 if on_content_delta and chunk.choices:
-                    text = getattr(chunk.choices[0].delta, "reasoning_content", None)
-                    if text:
-                        await on_content_delta(text)
                     text = getattr(chunk.choices[0].delta, "content", None)
                     if text:
                         await on_content_delta(text)