Add deprecation warning NETSDK1234 for RoslynCompilerType=Framework

[Copilot]

Part of dotnet/roslyn#81184.

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Plan for Adding Deprecation Warning for RoslynCompilerType=Framework

• Add new warning message string (NETSDK1234) to Strings.resx
• Create deprecation check for RoslynCompilerType
• Add comprehensive tests for the warning
  • Test that warning appears when RoslynCompilerType=Framework is set
  • Test that warning can be suppressed with NoWarn=NETSDK1234
  • Test that warning does not appear for other RoslynCompilerType values
• Build and test the changes
• Address PR feedback

Changes Made

PR Feedback Addressed

• ✅ Inlined the deprecation target directly into Microsoft.NET.Sdk.BeforeCommon.targets (removed separate file)
• ✅ Changed URL from aka.ms link to direct GitHub issue: Deprecate RoslynCompilerType=Framework roslyn#81184
• ✅ Updated test file to use file-scoped namespace
• ✅ Added sealed keyword to test class
• ✅ Converted to primary constructor
• ✅ Removed localized message text assertion (only checking for NETSDK1234 code)
• ✅ Added locked attributes to Strings.resx for RoslynCompilerType, Framework, and {0}
• ✅ Removed WarningsAsErrors test (SDK warnings don't work with TreatWarningsAsErrors the same way)
• ✅ Fixed test search pattern to use " NETSDK1234" (with leading space) to avoid matching /nowarn:NETSDK1234 in command line output on Windows

Implementation

• Warning Message (Strings.resx): NETSDK1234 with proper locked attributes
• Target (inlined in Microsoft.NET.Sdk.BeforeCommon.targets): Checks for deprecated value and emits warning
• Tests (6 tests, all passing ✅): Comprehensive coverage of warning and suppression scenarios

All tests passing: 6/6 ✅

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[333fred]

Do we normally use github links? Or some kind of shortlink?