[MTP Reporters Analysis] HtmlReport rejects path-valued --report-html-filename #8413

Summary

HtmlReport rejects path-valued --report-html-filename arguments even though the HTML engine already writes through Path.Combine(outputDirectory, fileName). This makes HTML inconsistent with TRX and blocks report-path customization for the HTML reporter.

Evidence

• src/Platform/Microsoft.Testing.Extensions.HtmlReport/HtmlReportGeneratorCommandLine.cs:45-50

  if (!IsValidPureFileName(fileName))
  {
      return ValidationResult.InvalidTask(ExtensionResources.HtmlReportFileNameShouldNotContainPath);
  }

• src/Platform/Microsoft.Testing.Extensions.HtmlReport/HtmlReportEngine.cs:65-70

  string fileName = fileNameExplicitlyProvided
      ? providedFileName![0]
      : BuildDefaultFileName(finishTime);
  
  string outputDirectory = _configuration.GetTestResultDirectory();
  string finalPath = Path.Combine(outputDirectory, fileName);

• For comparison, src/Platform/Microsoft.Testing.Extensions.TrxReport/TrxCommandLine.cs:45-48 explicitly allows relative or absolute paths for --report-trx-filename.

Why this is a real issue

A command like --report-html --report-html-filename subdir\report.html fails validation today, while the analogous TRX scenario is supported. The HTML engine would already place a relative path under the results directory (and Path.Combine would also honor rooted paths), so the current restriction is a CLI-only inconsistency that blocks a concrete customization scenario.

Suggested resolution

Align --report-html-filename with TRX semantics:

• allow relative and absolute paths,
• sanitize/validate only the leaf file name,
• create missing intermediate directories before writing.

Related issues

• TRX file name output should support path #5634
• Microsoft.Testing.Extensions.TrxReport should allow customizing trx file name via dedicated MSBuild property #6648
• Unify Configuration/Options #6349