Fix ProjectMigrateErrorMessage to correctly describe packages.config to PackageReference migration

[Copilot]

Summary

Fixes the misleading error message shown when a project is not eligible for migration from packages.config to PackageReference.

Problem

The error message ProjectMigrateErrorMessage incorrectly stated:

"Project is not eligible for migration as it's not project.json based."

This message was confusing and incorrect because:

1. The migration feature is for upgrading packages.config projects to PackageReference, not for project.json projects
2. Project.json has been deprecated and is no longer recommended
3. The message suggested users should use project.json, which contradicts current best practices

Solution

Updated the error message to accurately describe the migration requirements:

"Project is not eligible for migration. Either the project is not packages.config based or doesn't support PackageReference yet."

This message now:

• Correctly identifies that the project must be packages.config based
• Explains why migration might not be available (project doesn't support PackageReference)

Context

The message is displayed in Visual Studio when users attempt to migrate a project using the "Migrate packages.config to PackageReference..." command, but the project is not eligible. The underlying code (NuGetProjectUpgradeUtility.IsNuGetProjectUpgradeableAsync) specifically checks for packages.config projects that support PackageReference, confirming this is a packages.config → PackageReference migration, not a project.json migration.

Fixes #14099

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Migrator (PC to PR) shouldn't suggest using project.json project</issue_title>
<issue_description>### NuGet Product Used

Visual Studio Package Management UI

Product Version

any

Worked before?

No response

Impact

None

Repro Steps & Context

The error, 'Project is not eligible for migration as it's not project.json based.', is incorrectly suggesting to use project.json projects.

ProjectMigrateErrorMessage should be revised.

NuGet.Client/src/NuGet.Clients/NuGet.Tools/Resources.resx

Line 153 in ab6b96f

<data name="ProjectMigrateErrorMessage" xml:space="preserve">

Verbose Logs

```</issue_description>

<agent_instructions>This should only be relevant to packages.config projects, not project.json projects. In fact, we have deprecated project.json. Can the string resource be updated accordingly?</agent_instructions>

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

<comments>
</comments>

Fixes NuGet/Home#14240

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[donnie-msft]

@copilot please remove the URL portion and it's good to go

[Copilot]

@copilot please remove the URL portion and it's good to go

Done! Removed the URL portion from the error message in commit cc51c6d.