Simplify file-level directive diagnostics

[jjonescz]

Fixes #49817.

[Copilot]

Pull Request Overview

This PR simplifies file-level directive diagnostics by removing redundant location information from error messages and updating the error reporting format. The changes centralize location formatting and eliminate duplication where file path and line numbers were included in both the diagnostic location and message text.

• Refactored directive error messages to exclude file path and line number parameters
• Updated error reporting to use a consistent format with location followed by "error:" prefix
• Modified test infrastructure to work with the new diagnostic format

Reviewed Changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test/dotnet.Tests/CommandTests/Run/RunFileTests.cs	Added DirectiveError helper method and updated test assertions to use new error format
test/dotnet.Tests/CommandTests/Project/Convert/DotnetProjectConvertTests.cs	Updated test methods to use new error format and modified expected error structures
src/Cli/dotnet/Commands/Run/VirtualProjectBuildingCommand.cs	Simplified diagnostic reporting by removing location lambdas and updating error message format
src/Cli/dotnet/Commands/CliCommandStrings.resx	Updated error message strings to remove location parameters and added new DirectiveError string
Multiple .xlf files	Updated localization files with new error message formats and added DirectiveError translation entries

[jjonescz]

@RikkiGibson @333fred for reviews of a simple PR, thanks