fix(openai): use ReasoningContent for reasoning stream Start events

convert_openai_to_agentex_events opened reasoning summary/content messages with
a TextContent Start. On the migrated auto_send/Temporal path this regressed the
prior behavior (which started reasoning with ReasoningContent), so consumers
branching on the start event's content type render reasoning as plain text. The
final persisted content is rebuilt from the reasoning deltas regardless, so this
only affects the live stream envelope. Aligns with the codex/claude_code taps
and the langgraph-sync converter, and matches what the openai conformance suite
already treats as canonical.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: declan-scale

src/agentex/lib/adk/providers/_modules/sync_provider.py

@@ -32,6 +32,7 @@
 from agentex import AsyncAgentex
 from agentex.lib.utils.logging import make_logger
 from agentex.lib.core.tracing.tracer import AsyncTracer
+from agentex.types.reasoning_content import ReasoningContent
 from agentex.types.task_message_delta import TextDelta
 from agentex.types.task_message_update import (
     StreamTaskMessageDone,
@@ -560,14 +561,20 @@ async def convert_openai_to_agentex_events(stream_response):
                         item_id_to_index[item_id] = message_index
                         item_id_to_type[item_id] = "reasoning_summary"
 
-                        # Send a start event for this new reasoning summary message
+                        # Send a start event for this new reasoning summary message.
+                        # The start content must be ReasoningContent (not TextContent)
+                        # so consumers that branch on the start event's content type
+                        # render a reasoning/thinking indicator; the final persisted
+                        # content is rebuilt from the reasoning deltas regardless.
                         yield StreamTaskMessageStart(
                             type="start",
                             index=item_id_to_index[item_id],
-                            content=TextContent(
-                                type="text",
+                            content=ReasoningContent(
+                                type="reasoning",
                                 author="agent",
-                                content="",  # Start with empty content
+                                summary=[],
+                                content=[],
+                                style="active",
                             ),
                         )
 
@@ -604,14 +611,20 @@ async def convert_openai_to_agentex_events(stream_response):
                         item_id_to_index[item_id] = message_index
                         item_id_to_type[item_id] = "reasoning_content"
 
-                        # Send a start event for this new reasoning content message
+                        # Send a start event for this new reasoning content message.
+                        # The start content must be ReasoningContent (not TextContent)
+                        # so consumers that branch on the start event's content type
+                        # render a reasoning/thinking indicator; the final persisted
+                        # content is rebuilt from the reasoning deltas regardless.
                         yield StreamTaskMessageStart(
                             type="start",
                             index=item_id_to_index[item_id],
-                            content=TextContent(
-                                type="text",
+                            content=ReasoningContent(
+                                type="reasoning",
                                 author="agent",
-                                content="",  # Start with empty content
+                                summary=[],
+                                content=[],
+                                style="active",
                             ),
                         )