Package Source Mapping VS Settings now has command to invoke GitHub Copilot

[donnie-msft]

Bug

Fixes: https://github.com/NuGet/Client.Engineering/issues/3717

Description

Introduce a Button (Command) to the Package Source Mapping VS Settings which attempts to prompt GitHub Copilot with "Review this repository's package source mappings.", invoking the NuGet MCP tool to review supply chain security.

The new button appears below the package source mapping array, highlighted here by a red rectangle:

• In this iteration, the button is always shown. If Package Source Mappings exist in a machine-level NuGet.Config, the MCP Tool will suggest and help to move those into repo-level NuGet.Config. It can also help simplify any existing mappings.

• A PackageSourceMappingService is added, closely mimicking the FixVulnerablitiesService.

• Refactored FixVulnerablitiesService:

  • Extract HandleSessionError on CopilotToolInvocationService as a shared static method for error messaging + telemetry, used by both PSM and FixVulnerabilities services
  • Replace FixVulnerabilitiesWithCopilotErrorType with CopilotToolSessionError (moved to NuGet.PackageManagement.VisualStudio/Telemetry/) — single enum for both session errors and telemetry

• Add a NavigatedTelemetryEvent event with matching test coverage

• Add PackageSourceMapperCommand for the VS Options button wiring

PR Checklist

• Meaningful title, helpful description and a linked NuGet/Home issue
• Added tests
• Link to an issue or pull request to update docs if this PR changes settings, environment variables, new feature, etc.

[donnie-msft]

Moving to draft while we discuss the overall design.