[WIP] Fix issue 37230 in EF Core

[Copilot]

• Understand issue dotnet ef migrations script gives me error MSB4057: The target "ResolvePackageAssets" does not exist in the project. #37230: The dotnet ef commands fail with "MSB4057: The target 'ResolvePackageAssets' does not exist in the project" when the project uses plural TargetFrameworks (multi-targeting)
• Analyze the root cause: In RootCommand.cs line 71, Project.FromFile is called without passing the _framework option for the main project, while the startup project (lines 72-77) correctly receives this parameter
• Apply fix: Pass _framework, _configuration, and _runtime options to the main project's Project.FromFile call in RootCommand.cs
• Add helpful error message when multi-targeting is detected without a framework specified
• Add MultipleTargetFrameworks resource string
• Add helper method HasMultipleTargetFrameworks to detect multi-targeting projects
• Build and test the changes
• Run code review

Original prompt

Fix #37230

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