Give clearer errors when Microsoft.dotnet-msidentity is not installed

[haileymck]

Summary

When the Microsoft.dotnet-msidentity tool was missing, Entra ID scaffolding failed with cryptic errors. RegisterAppStep invoked dotnet msidentity directly with no install/detection check, so a missing tool surfaced as a raw dotnet "command not found" error. AddClientSecretStep had its own duplicated install logic but no user-facing guidance on how to fix a failure.

This PR centralizes the detection/install logic and makes every failure path emit clear, actionable guidance.

Changes

• New Helpers/MsIdentityToolHelper.cs — shared helper exposing:
  • IsMsIdentityInstalled / EnsureMsIdentityIsInstalled (lifted from AddClientSecretStep, now reusable).
  • NotInstalledMessage — a single actionable message telling the user exactly how to install the tool:

    The 'Microsoft.dotnet-msidentity' tool is required for Entra ID scaffolding but is not installed or could not be found. Install it by running 'dotnet tool install --global Microsoft.dotnet-msidentity' (append '--prerelease' when using a prerelease build), then run the scaffolder again.

• RegisterAppStep.cs — now calls EnsureMsIdentityIsInstalled up front (it previously had no check at all) and appends NotInstalledMessage on register/update CLI failures.
• AddClientSecretStep.cs — removed the duplicated EnsureMsIdentityIsInstalled/IsMsIdentityAlreadyInstalled methods in favor of the shared helper, and surfaces NotInstalledMessage on CLI failure.

Tests

• Added MsIdentityToolHelperTests (7 tests) verifying the tool name/command constants and that NotInstalledMessage includes the install command, prerelease guidance, and Entra ID context. All pass.
• Existing RegisterAppStep/AddClientSecretStep tests still compile and build cleanly.

Notes

The repo pins an SDK (11.0.100-preview.6) that isn't installed on the build machine, so builds/tests were verified against the installed preview.5 via a temporary global.json swap (reverted afterward — global.json is unchanged in this PR).