Fix Assistants IChatClient handling of unrelated tool calls in history

[stephentoub]

The implementation is assuming that any tool results in the incoming message history mean a thread ID is required, which would be necessary if those tool results were for an active run. But that's not the case if the history just contains historical function calls/responses, as is the case in agent scenarios where the message history from one agent is passed to another. The fix is to just stop throwing based on this incorrect assumption.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull Request Overview

This PR fixes a bug in the OpenAI Assistants IChatClient implementation where it incorrectly assumed that any tool results in message history required a thread ID. The fix addresses scenarios where message history contains historical function calls/responses (like in agent scenarios) rather than active tool calls requiring a thread.

Key changes:

• Removes validation that incorrectly throws when tool results are present without a thread ID
• Updates logic to only process function results when the message role is Tool
• Improves comments to clarify the distinction between active and historical tool results