[Editor] Prevent `TOOLS` .Net DefineConstants being overriden by the user

[Delsin-Yu]

Related #78513 Closes #98124

As mentioned here, the TOOLS compiler symbol is crucial for C# editor functionality; the absence of TOOLS can lead to data loss and editor malfunctions (all C# script exports become null and Ineditable) without a proper error message.

This PR ensures that the TOOLS are always included in the editor, so even if the user overrides the DefineConstants (by accident or intentionally), the rest of the dotnet editor stays functional.

Thanks Raulsntos for suggesting the better solution.