feat: log SQ report file path and size to console after generation

[Copilot]

When the SonarQube report is written, log the file path and size (in bytes) to the console.

Example output:

SonarQube report written to: /path/to/report.json (1234 bytes)

Changes:

• Added GetFileSize(string path) to IFileSystem interface
• Implemented in RealFileSystem using new FileInfo(path).Length
• Implemented in both MockFileSystem test helpers (returns MemoryStream.Length)
• Injected IFileSystem into ValidatorRunner and logs after WriteReportAsync completes
• Updated existing tests to pass fileSystem to the ValidatorRunner constructor