Skip obj→bin copy on build failure in CSharpCompilerCommand

[Copilot]

CSharpCompilerCommand.Execute() was copying the obj dll to bin regardless of build success, causing unnecessary work and potential errors when the dll doesn't exist due to compilation failures.

Changes

• CSharpCompilerCommand.cs: Gate the obj→bin copy with exitCode == 0 check
• RunFileTests.cs: Add CscOnly_CompilationFailure_NoCopyToBin test verifying bin dll is not updated after failed compilation

The fix ensures failed builds don't produce or modify output artifacts:

// Process the response.
var exitCode = ProcessBuildResponse(responseTask.Result, out fallbackToNormalBuild);

// Copy from obj to bin only if the build succeeded.
if (exitCode == 0 &&
    BuildResultFile != null &&
    CSharpCommandLineParser.Default.Parse(...) is { OutputFileName: { } outputFileName } parsedArgs)
{
    // ... copy logic
}

Original prompt

Let's modify CSharpCompilerCommand to skip copying the obj dll to bin directory if the build fails (it's an unnecessary work and can lead to spurious errors, for example if the source file was not even produced by roslyn due to compilation errors - add a test for that too)

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[Copilot]

Pull Request Overview

This PR adds a safeguard to prevent copying DLL files from the obj directory to the bin directory when C# compilation fails. The change ensures that spurious errors don't occur from attempting to copy non-existent or incomplete DLL files after compilation errors.

Key Changes

• Modified the copy logic in CSharpCompilerCommand.cs to only copy files when exitCode == 0
• Added comprehensive test coverage for the compilation failure scenario

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Cli/dotnet/Commands/Run/CSharpCompilerCommand.cs	Added exit code check to prevent copying DLLs when compilation fails
test/dotnet.Tests/CommandTests/Run/RunFileTests.cs	Added test to verify DLLs are not copied when compilation fails

[RikkiGibson]

Is there a specific issue that is being solved by this PR? If so, please link to it.

[jjonescz]

There is not an existing issue afaik; I noticed the problem while working on another PR. The added test has the "repro" for the issue though.