Fix HTML logger parallel file collision with atomic file creation

[nohwnd]

Summary

Fix cross-process race condition in HTML logger file creation that causes collisions when multiple test processes run in parallel.

This is a cleaned-up version of #15435.

Changes

• Atomic file creation: Replace lock + File.Exists check with FileMode.CreateNew in GenerateUniqueFilePath. The lock only protected within a single process; FileMode.CreateNew is atomic at the OS level across all processes.
• Fractional-second timestamps: Add fractional seconds (\ffffff) to the timestamp format in both \FormatDateTimeForRunName\ and the \LogFilePrefix\ path, providing better uniqueness when multiple processes start near-simultaneously.
• Remove dead code: Remove the now-unnecessary static \FileCreateLockObject.
• Add test: Verify the generated filename contains the fractional-seconds timestamp pattern.

Closes #15404

[Copilot]

Pull request overview

Fixes cross-process collisions in the HTML logger’s output file creation by switching to OS-level atomic file creation and increasing timestamp granularity.

Changes:

• Use FileMode.CreateNew in GenerateUniqueFilePath to avoid cross-process races.
• Add fractional-second timestamps (fffffff) to output naming for better uniqueness.
• Add a unit test asserting the fractional-second filename pattern.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
test/Microsoft.TestPlatform.Extensions.HtmlLogger.UnitTests/HtmlLoggerTests.cs	Adds coverage for the new fractional-second timestamp filename format.
src/Microsoft.TestPlatform.Extensions.HtmlLogger/HtmlLogger.cs	Implements atomic file creation and updates timestamp formats used in generated filenames.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.