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.