.Net - Agents: chat.Add*(...) => chat.Add(...)

[crickman]

Motivation and Context

Exploring alignment with other SK conventions

Description

Contribution Checklist

• [X] The code builds clean without any errors or warnings
• [X] The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• [X] All unit tests pass, and I have added new tests where possible
• [X] I didn't break anyone :smile: