Make FileLogger thread-safe

[Nils Peper (MSFT) (nilspep)]

📖 Description

FileLogger writes to a single std::ofstream without synchronization. When a diagnostic logger is shared across threads, concurrent Write / WriteDirect calls can interleave inside one record, and the maximum-size wrap (HandleMaximumFileSize / WrapLogFile) can race with other writers. This can produce merged or torn log lines.

Upstream code already writes to the same context logger from other threads. For example, winget configure routes configuration processor diagnostics (processor.Diagnostics(...) in ConfigurationFlow.cpp) and WinRT progress callbacks into the context's diagnostic logger.

This change adds a per-instance mutex that serializes:

• WriteDirect, which Write uses; this covers the size check, wrapping and the stream write;
• SetTag(HeadersComplete);
• SetMaximumSize.

The mutex is held through a std::unique_ptr so that FileLogger stays movable. Single-threaded behavior and output format are unchanged.

🔗 References

Resolves #6570

🔍 Validation

• New test FileLogger_ConcurrentWritesPreserveCompleteLines: 8 threads write 1,000 records each, formatted and direct, with and without a maximum file size. Every line in the file must be exactly one complete record.
  • Without the fix, it failed in 3 of 3 runs on an x64 Release build (torn or merged lines).
  • With the fix, it passed in 3 of 3 runs.
• The complete AppInstallerCLITests suite passes on x64 Release with the fix (1,123 test cases).

✅ Checklist

• Signed the Contributor License Agreement
• Linked to an issue
• Updated Release Notes (if applicable)
• Updated documentation (if applicable)
• Added or updated tests (or noted why not applicable)
• Updated Copilot instructions (if build, architecture, or conventions changed)

🤖 AI Assistance

• AI assistance was used and has been disclosed in this PR

Fix designed by me; GitHub Copilot assisted with implementation, testing and validation under my direction.

📋 Issue Type

• Bug fix
• Feature
• Task

Microsoft Reviewers: Open in CodeFlow

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