Fix Copilot SDK integration

[Redth]

The Copilot integration was broken because the app was still using an older SDK that cannot deserialize responses from the current Copilot CLI. This updates the SDK usage and fixes the connected-but-sessionless path exposed during native macOS validation.

Changes

• Update GitHub.Copilot.SDK references to 1.0.2 for Core and native macOS.
• Migrate CopilotService to the v1 SDK APIs for stdio connections, session events, tool declarations, and permission decisions.
• Ensure a Copilot session is started before sending when availability checks have already created/reused a connected client.

Validation

• Built native macOS target: dotnet build src/MauiSherpa.MacOS/MauiSherpa.MacOS.csproj -f net10.0-macos -v minimal
• Ran Core tests: dotnet test tests/MauiSherpa.Core.Tests/MauiSherpa.Core.Tests.csproj -v minimal
• Validated native macOS Copilot flow: toolbar Copilot opens, authenticates as Redth, starts a session, sends a prompt, and receives an assistant response.

Notes

• This intentionally does not change Mac Catalyst. Native macOS/AppKit was used for final validation.