Fix case-sensitivity probe corrupting directory path in MergeOutputFileHelper

[Evangelink]

Summary

Follow-up bug fix for the post-merge review comment on #9805: #9805 (comment)

MergeOutputFileHelper.IsDirectoryCaseSensitive(string directory) probed case sensitivity by lower-casing the entire candidate path:

string upper = Path.Combine(directory, "CASESENSITIVEPROBE" + Guid.NewGuid().ToString("N"));
using (new FileStream(upper, ...))
{
    return !File.Exists(upper.ToLowerInvariant());
}

upper.ToLowerInvariant() also lower-cases the directory portion. For a case-insensitive/case-folded child directory (e.g. an ext4 casefold dir named with UPPERCASE chars) sitting beneath a case-sensitive parent, the lower-cased parent no longer exists, so File.Exists returns false and the method wrongly reports the directory as case-sensitive (Ordinal).

That makes EnsureOutputDoesNotAliasInput use StringComparison.Ordinal, so it can miss that a.trx and A.trx are the same file, and the subsequent ReplaceFile can overwrite an input that RFC 018 requires to stay read-only (data loss).

Fix

Lower-case only the generated probe file name, preserving the real directory path, and expand the comment to explain why.

Test

Adds a regression test MergeToFileAsync_WhenOutputDiffersFromInputOnlyByCase_IsRejectedOnCaseInsensitiveDirectory in CtrfReportMergerTests.cs that goes through the public MergeToFileAsync surface: on a case-insensitive directory a case-only output (report.json input vs REPORT.json output) must be rejected as an alias; on a genuinely case-sensitive directory the two names are distinct and the merge proceeds. The directory's real case sensitivity is probed independently (correctly, lower-casing only the file name).

Validation (Windows)

• 3 src projects build 0 warnings / 0 errors with default analyzer enforcement: Microsoft.Testing.Extensions.TrxReport, Microsoft.Testing.Extensions.CtrfReport, Microsoft.Testing.Extensions.JUnitReport.
• Test project builds 0/0 with /p:TreatWarningsAsErrors=true.
• Merge tests pass on net9.0 (79/79) and net462 (76/76).

[Copilot]

Pull request overview

Fixes case-sensitivity detection to prevent report mergers from overwriting aliased input files.

Changes:

• Preserves directory casing when probing filesystem sensitivity.
• Adds a case-only output alias test, though it does not reproduce the mixed-sensitivity regression.

Show a summary per file

File	Description
src/Platform/SharedExtensionHelpers/MergeOutputFileHelper.cs	Lowercases only the probe filename.
test/UnitTests/Microsoft.Testing.Extensions.UnitTests/CtrfReportMergerTests.cs	Adds filesystem case-sensitivity coverage.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Medium

[Copilot]

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0 new
• Review effort level: Medium

[Copilot]

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 0 new
• Review effort level: Medium

[github-actions]

🧪 Test quality grade — PR #10014

Grade	Test	Notes
C (70–79)	new CtrfReportMergerTests. BuildCaseFoldedProbePath_ LowerCasesOnlyFileName_ PreservingCaseSensitiveDirectory	Three solid equality assertions, but testing via BindingFlags.NonPublic reflection couples the test to the private implementation — consider exposing via internal + InternalsVisibleTo.
B (80–89)	new CtrfReportMergerTests. MergeToFileAsync_ WhenOutputAliasesInputByCaseOnly_ IsRejectedOnCaseInsensitiveFilesystem	Good end-to-end coverage with exception + file-preservation assertions; idiomatic Assert.Inconclusive skip for platform gating.
B (80–89)	new CtrfReportMergerTests. MergeToFileAsync_ WhenOutputDiffersByCaseOnly_ IsAllowedOnCaseSensitiveFilesystem	Clean complementary sibling; both-files-exist assertions verify the key postcondition.

_{This advisory comment was generated automatically. Grades are heuristic
and informational — they do not block merging. Re-run with
/grade-tests.}

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