PerfDiff: Fix incorrect conditional logic in file existence checks

[coderabbitai]

Description

Logic errors in file existence validation that cause incorrect behavior when checking file paths and extensions.

Issues Identified from PR #565

Incorrect Conditional Logic

• PerfDiff.TryGetETLPaths(): Uses || instead of && in file existence check
• BenchmarkDotNetDiffer: Similar logic error in file validation
• Current logic incorrectly returns true for any existing file regardless of extension

Affected Files

• src/tools/PerfDiff/PerfDiff.cs (lines 104-108)
• src/tools/PerfDiff/BDN/BenchmarkDotNetDiffer.cs

Required Fix

-        else if (File.Exists(path) || !path.EndsWith(ETLFileExtension, StringComparison.OrdinalIgnoreCase))
+        else if (File.Exists(path) && path.EndsWith(ETLFileExtension, StringComparison.OrdinalIgnoreCase))

Impact

• Files with incorrect extensions may be processed incorrectly
• ETL trace analysis may fail silently
• Performance regression detection may produce false results

Reference

• Original PR: feat: modularize PerfDiff regression detection, update analyzer reporting, and clarify CI performance documentation #565
• Review comments: feat: modularize PerfDiff regression detection, update analyzer reporting, and clarify CI performance documentation #565 (comment)