fix: override end_turn stop reason when streaming response contains toolUse blocks

[atian8179]

Problem

Some models (e.g., Sonnet 4.x on Bedrock) return end_turn as the stop reason in messageStop even when the response contains toolUse content blocks. In streaming mode, the stop_reason from messageStop is used directly without checking for toolUse blocks, causing the event loop to skip tool execution entirely.

This results in the agent returning prematurely with unexecuted tool calls.

Root Cause

In stream_messages_async() (streaming.py), the stop_reason from handle_message_stop() is yielded as-is. Unlike the non-streaming path in event_loop.py (which checks _has_tool_use_in_latest_message), the streaming path has no corresponding override.

Fix

After the streaming loop completes, check if the assembled message content contains toolUse blocks. If stop_reason is end_turn but toolUse blocks are present, override to tool_use.

Fixes #1810

[pgrayy]

Few call outs:

1. I wouldn't worry about mentioning the ticket here in comments. That is more for the PR itself.
2. We should try to fix this before the yield ModelStreamChunkEvent above. This way the event is yielded with the correct stop reason.
3. This I would say warrants a warn instead of a debug.
4. With this change, we should be able to remove the override logic in https://github.com/strands-agents/sdk-python/blob/main/src/strands/models/bedrock.py#L826.
5. We should add tests.