Fix FileScanner crash on Parallel.ForEach AggregateException

Wrap both the parallel and sequential subdirectory loops with try/catch
for IOException, UnauthorizedAccessException, and PathTooLongException.
Also catch AggregateException from Parallel.ForEach to prevent crashes
when scanning directories with problematic paths (symlink loops, network
mounts timing out, paths exceeding OS limits).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: HavenDV

src/cli/Tiktoken.Cli/IO/FileScanner.cs

@@ -331,13 +331,31 @@ private void ScanDirectory(
         if (depth <= 1 && subDirs.Count > 1)
         {
             var bags = new ConcurrentBag<(List<string> Results, ScanStats Stats)>();
-            Parallel.ForEach(subDirs, subDir =>
+            try
             {
-                var localResults = new List<string>();
-                var localSubStats = new ScanStats();
-                ScanDirectory(subDir, rootPath, rootPrefixLen, effectiveIgnores, effectiveHasFileRules, localResults, localSubStats, depth + 1);
-                bags.Add((localResults, localSubStats));
-            });
+                Parallel.ForEach(subDirs, subDir =>
+                {
+                    var localResults = new List<string>();
+                    var localSubStats = new ScanStats();
+                    try
+                    {
+                        ScanDirectory(subDir, rootPath, rootPrefixLen, effectiveIgnores, effectiveHasFileRules, localResults, localSubStats, depth + 1);
+                    }
+                    catch (Exception ex) when (ex is IOException or UnauthorizedAccessException or PathTooLongException)
+                    {
+                        localSubStats.DirsErrored++;
+                    }
+                    bags.Add((localResults, localSubStats));
+                });
+            }
+            catch (AggregateException ae)
+            {
+                // If any worker threw an unhandled exception, count it and continue
+                foreach (var _ in ae.InnerExceptions)
+                {
+                    localStats.DirsErrored++;
+                }
+            }
 
             foreach (var (subResults, subStats) in bags)
             {
@@ -349,7 +367,14 @@ private void ScanDirectory(
         {
             foreach (var subDir in subDirs)
             {
-                ScanDirectory(subDir, rootPath, rootPrefixLen, effectiveIgnores, effectiveHasFileRules, results, localStats, depth + 1);
+                try
+                {
+                    ScanDirectory(subDir, rootPath, rootPrefixLen, effectiveIgnores, effectiveHasFileRules, results, localStats, depth + 1);
+                }
+                catch (Exception ex) when (ex is IOException or UnauthorizedAccessException or PathTooLongException)
+                {
+                    localStats.DirsErrored++;
+                }
             }
         }
     }