Update package display format from package::version to package@version

[Copilot]

• Explore repository structure and identify files to change
• Update InstallRequest.DisplayName to use @ instead of ::
• Update NuGetManagedTemplatePackage.DisplayName to use @ instead of ::
• Update test expectations in NuGetInstallerTests.cs to use @ instead of ::
• Update exception messages in NuGet exceptions to use @ instead of ::
• Update logging messages in NugetApiPackageManager.cs to use @ instead of ::
• Update logging messages in PackageManager.cs test helper to use @ instead of ::
• Update console output in TemplateDiscovery tools to use @ instead of ::
• Verify no other occurrences of package::version format remain
• Fix test expectations in TemplateDiscoveryTests for @ format
• Final review and validation

Summary of Changes

Updated all package identifier and version display formatting from the legacy package::version format to the new recommended package@version format across:

Core Properties:

• InstallRequest.DisplayName in Microsoft.TemplateEngine.Abstractions
• NuGetManagedTemplatePackage.DisplayName in Microsoft.TemplateEngine.Edge

Exception Messages:

• PackageNotFoundException constructor
• DownloadException constructors (3 overloads)

Logging Messages:

• NugetApiPackageManager.cs (5 locations)
• PackageManager.cs test helper (3 locations)

Console Output:

• AdditionalDataExtensions.cs in TemplateDiscovery tools
• PackSourceChecker.cs in TemplateDiscovery tools (12 locations)

Tests:

• NuGetInstallerTests.cs (4 test assertions)
• TemplateDiscoveryTests.cs (6 test assertions)

Total: 10 files modified, 37 changes across the codebase. All references consistently updated to use the @ symbol instead of :: for package@version formatting.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Update templating text to use @ instead of :: symbol</issue_title>
<issue_description>The package::version format is the legacy format and the recommendation now is to use @ instead. We have an SDK PR to update this but some of the strings come from the template engine as well. Both repos already support the new format so this is just a matter of ensuring all messaging recommends customers use the new format.

Here's the SDK PR: dotnet/sdk#51153

For the templating repo:

• Update InstallRequest.DisplayName
• Update TemplatePackage.DisplayName properties
• identify any additional changes needed to strings or tests related to this transition

Both DisplayNames come from the Microsoft.TemplateEngine.Edge package</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #9414

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[marcpopMSFT]

@copilot can you fix these three tests for the change in the :: output to @?

• CanDetectRemovedPackagesInDiffMode
• CanDetectUpdatedPackagesInDiffMode
• CanDetectNewPackagesInDiffMode

[Copilot]

@copilot can you fix these three tests for the change in the :: output to @?

• CanDetectRemovedPackagesInDiffMode
• CanDetectUpdatedPackagesInDiffMode
  ...

Fixed the three tests to expect @ instead of :: in the output. Updated 6 test assertions across:

• CanDetectNewPackagesInDiffMode (2 assertions)
• CanDetectUpdatedPackagesInDiffMode (1 assertion)
• CanDetectRemovedPackagesInDiffMode (3 assertions)

Commit: Will be included in the progress report.

[marcpopMSFT]

I looked through all instances of :: in the sources and this covers all the ones I would expect to update for this. I don't know how to test the template repo and am relying on the automated tests.

[MiYanni]

I can't verify if any of these changes are correct. But if it doesn't cause problems in the SDK repo, then it is likely fine. But we'd have to wait for this to flow there to verify that.