.NET: GitHub Copilot provider doesn't enforce `ApprovalRequiredAIFunction` for custom function tools

[giles17]

The .NET Microsoft.Agents.AI.GitHub.Copilot provider doesn't honor the Agent Framework approval boundary for custom function tools wrapped in ApprovalRequiredAIFunction. Because the GitHub Copilot SDK owns the tool-calling loop, the standard AF approval round-trip never runs, so the tool can execute without approval.

Direction

Rather than reimplementing enforcement, lean on the Copilot SDK's native OnPreToolUse hook (verified: it gates custom tools, runs before the permission layer, and "ask" routes to OnPermissionRequest):

• The provider installs a default OnPreToolUse hook only if the user hasn't supplied their own. It returns "ask" for tools wrapped in ApprovalRequiredAIFunction (routed to OnPermissionRequest to approve/deny) and defers (null) for everything else, so other tools surface through OnPermissionRequest as they do today.
• If the user supplies their own OnPreToolUse, theirs takes precedence and they own approval handling. This is documented, and the provider logs a warning when an approval-required tool would go unenforced.

A follow-up will align the Python provider to the same model.