[fix] Quote --diag path and harden AddDoubleQuote against embedded quotes/trailing backslashes

[github-actions]

🤖 This is an automated fix generated by the Issue Triage workflow.

Fixes #16449

Root cause

1. TestRunnerConnectionInfoExtensions.ToCommandLineOptions() embedded the user-supplied --diag log path directly into the shared options string without quoting it: $"{options} --diag {connectionInfo.LogFile} --tracelevel {connectionInfo.TraceLevel}". A diag path containing a space (e.g. a Windows working directory like C:\Users\Jane Doe\project) splits into multiple invalid arguments when the resulting string is used to launch the testhost/dotnet exec process.
2. StringExtensions.AddDoubleQuote() was a naive "\"" + value + "\"" wrapper: it didn't escape embedded " characters, and it didn't defend against the Windows CommandLineToArgvW trailing-backslash-before-closing-quote ambiguity (a path ending in \ immediately followed by the closing " is parsed as an escaped quote, corrupting the argument boundary — e.g. a drive root like D:\).

Fix

• TestRunnerConnectionInfoExtensions.ToCommandLineOptions() now calls AddDoubleQuote() on connectionInfo.LogFile before interpolating it into the --diag option.
• StringExtensions.AddDoubleQuote() now:
  • Escapes embedded " characters as \".
  • Doubles any run of trailing backslashes immediately preceding the closing quote, so paths like D:\ round-trip correctly.

This directly addresses Task 1 (High priority) and Task 2 (Medium priority) from the linked quality-focus issue (#16449). Task 3 (migrating to ProcessStartInfo.ArgumentList) is a larger/architectural change better suited for separate follow-up work, and is not attempted here.

Tests

• test/Microsoft.TestPlatform.ObjectModel.UnitTests/Hosting/TestRunnerConnectionInfoExtensionsTests.cs: updated the existing diag-option test to expect the quoted path, and added a new test asserting a LogFile path containing spaces is quoted correctly.
• test/Microsoft.TestPlatform.CoreUtilities.UnitTests/Extensions/StringExtensionsTests.cs (new file): covers plain values, values with spaces, embedded double quotes, single/multiple trailing backslashes, and backslashes not at the end of the string.

All targeted tests pass:

Microsoft.TestPlatform.CoreUtilities.UnitTests (StringExtensionsTests): 6/6 passed
Microsoft.TestPlatform.ObjectModel.UnitTests (TestRunnerConnectionInfoExtensionsTests): 7/7 passed

No public API surface was added (AddDoubleQuote already existed), so PublicAPI.Unshipped.txt files are unchanged.

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🔍 Triaged by Issue Repro Triage & Auto-Fix 🔍

---

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Show patch preview (162 of 162 lines)

From f819f40bf205096dca31835715834f0cea659181 Mon Sep 17 00:00:00 2001
X-GH-AW-Base-Commit: 1f0a9175d5dc8c95232e80ab538149c79f13597c
From: "github-actions[bot]" <github-actions[bot]@users.noreply.github.com>
Date: Wed, 9 Sep 2026 01:13:42 +0000
Subject: [PATCH] Quote --diag path and harden AddDoubleQuote against embedded
 quotes/trailing backslashes

Fixes #16449

- TestRunnerConnectionInfoExtensions.ToCommandLineOptions now quotes the
  --diag log file path, so paths containing spaces don't split into
  multiple invalid testhost command-line arguments.
- StringExtensions.AddDoubleQuote now escapes embedded double quotes and
  doubles any run of trailing backslashes immediately before the closing
  quote, avoiding the classic CommandLineToArgvW trailing-backslash
  ambiguity for paths like drive roots (D:\).
- Added unit tests for both changes.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 .../Extensions/StringExtensions.cs            | 20 +++++++-
 .../TestRunnerConnectionInfoExtensions.cs     |  3 +-
 .../Extensions/StringExtensionsTests.cs       | 47 +++++++++++++++++++
 ...TestRunnerConnectionInfoExtensionsTests.cs | 12 ++++-
 4 files changed, 79 insertions(+), 3 deletions(-)
 create mode 100644 test/Microsoft.TestPlatform.CoreUtilities.UnitTests/Extensions/StringExtensionsTests.cs

diff --git a/src/Microsoft.TestPlatform.CoreUtilities/Extensions/StringExtensions.cs b/src/Microsoft.TestPlatform.CoreUtilities/Extensions/StringExtensions.cs
index c47ab59..2f2d6a8 100644
--- a/src/Microsoft.TestPlatform.CoreUtilities/Extensions/StringExtensions.cs
+++ b/src/Microsoft.TestPlatform.CoreUtilities/Extensions/StringExtensions.cs
@@ -7,11 +7,29 @@ public static class StringExtensions
 {
     /// <summary>
     /// Add double quote around string. Useful in case of path which has white space in between.
+    /// Embedded double quotes are escaped and any run of trailing backslashes immediately
+    /// preceding the closing quote is doubled, so
... (truncated)