Fix async signature

[crickman]

Motivation and Context

Incorrect controller route/method signature causing build error

Description

D:\a\chat-copilot\chat-copilot\webapi\Controllers\MaintenanceController.cs(50,55): warning CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread. [D:\a\chat-copilot\chat-copilot\webapi\CopilotChatWebApi.csproj]

Contribution Checklist

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