Fix a couple of issues in M.E.AI.OpenAI clients

[stephentoub]

1. Setting AllowMultipleToolCalls if there aren't any tools results in failure. Fix it to only set the property if tools have been added.
2. The chat completion service fails if a participant name containing anything other than a constrained set of characters are included. Fix it with a sanitized value.

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull Request Overview

This PR fixes two issues in the Microsoft.Extensions.AI.OpenAI clients: preventing failures when setting AllowMultipleToolCalls without tools, and handling participant names with invalid characters through sanitization.

• Conditionally set AllowMultipleToolCalls only when tools are present to prevent API failures
• Implement author name sanitization to remove invalid characters for OpenAI participant names
• Add comprehensive test coverage for the participant name sanitization functionality

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
OpenAIChatClient.cs	Adds author name sanitization logic and conditional AllowMultipleToolCalls setting
OpenAIResponsesChatClient.cs	Moves AllowMultipleToolCalls setting to only apply when tools exist
OpenAIConversionTests.cs	Updates tests to verify participant name sanitization behavior
CHANGELOG.md	Documents the bug fixes in the release notes

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}