ExtractionExceptions is a non-thread-safe List<string> accessed from concurrent threads

[imnasnainaec]

Part of #135.

ExtractionExceptions is declared as static List<string>. When ExtractStringsFromCode is called from concurrent threads, multiple threads may call AddRange on the same list simultaneously, causing data corruption.

Fix: Change ExtractionExceptions from List<string> to ConcurrentBag<string> and replace AddRange calls with individual Add calls.

Files affected: src/L10NSharp/XLiffUtils/XliffLocalizationManager.cs

[imnasnainaec]

Claude fix summary:

Affected file: src/L10NSharp/XLiffUtils/XliffLocalizationManager.cs

• Line 219: public static List<string> ExtractionExceptions = new List<string>();
• Line 231: ExtractionExceptions.AddRange(extractor.ExtractionExceptions);

Fix: Change to ConcurrentBag<string> (already used elsewhere in the codebase):

public static ConcurrentBag<string> ExtractionExceptions = new ConcurrentBag<string>();
// ...
foreach (var ex in extractor.ExtractionExceptions)
    ExtractionExceptions.Add(ex);

Requires using System.Collections.Concurrent; if not already present.

Fix size: XS | Review: Easy | Risk: Low (but breaking API)