fix(agent): ensure first delta is fired during reasoning updates

- Added calls to `_fire_first_delta()` in the `AIAgent` class to ensure that the first delta is triggered for both reasoning and thinking updates. This change improves the handling of delta events during streaming, enhancing the responsiveness of the agent's reasoning capabilities.

run_agent.py

@@ -3616,6 +3616,7 @@ class AIAgent:
                 reasoning_text = getattr(delta, "reasoning_content", None) or getattr(delta, "reasoning", None)
                 if reasoning_text:
                     reasoning_parts.append(reasoning_text)
+                    _fire_first_delta()
                     self._fire_reasoning_delta(reasoning_text)
 
                 # Accumulate text content — fire callback only when no tool calls
@@ -3730,6 +3731,7 @@ class AIAgent:
                             elif delta_type == "thinking_delta":
                                 thinking_text = getattr(delta, "thinking", "")
                                 if thinking_text:
+                                    _fire_first_delta()
                                     self._fire_reasoning_delta(thinking_text)
 
                 # Return the native Anthropic Message for downstream processing