Fix FileSnippet test race and retry the unchanged-content read

src/MarkdownSnippets/Processing/DirectoryMarkdownProcessor.cs

@@ -340,14 +340,36 @@ void WriteLines(string target, List<Line> lines)
         // and reduces the chance of colliding with another process reading the file
         // (e.g. a parallel build packing a nuget that consumes the same markdown).
         if (File.Exists(target) &&
-            File.ReadAllText(target) == content)
+            ReadAllTextWithRetry(target) == content)
         {
             return;
         }
 
         WriteAllTextWithRetry(target, content);
     }
 
+    static string? ReadAllTextWithRetry(string target)
+    {
+        const int maxAttempts = 5;
+        for (var attempt = 1; ; attempt++)
+        {
+            try
+            {
+                return File.ReadAllText(target);
+            }
+            catch (IOException) when (attempt < maxAttempts)
+            {
+                // Another process is writing the file. Back off briefly and retry.
+                Thread.Sleep(100 * attempt);
+            }
+            catch (IOException)
+            {
+                // Still locked: treat as changed and let the write path deal with it
+                return null;
+            }
+        }
+    }
+
     static void WriteAllTextWithRetry(string target, string content)
     {
         const int maxAttempts = 5;

src/Tests/DirectoryMarkdownProcessor/FileSnippetExplicitInclude/one.source.md

@@ -0,0 +1,12 @@
+Local
+
+snippet: sourceFile.txt
+
+Relative Local
+
+snippet: ./sourceFile.txt
+
+Rooted
+
+snippet: /sourceFile.txt
+

src/Tests/DirectoryMarkdownProcessor/FileSnippetExplicitInclude/sourceFile.txt

@@ -0,0 +1 @@
+From Source File

src/Tests/DirectoryMarkdownProcessorTests.cs

@@ -391,7 +391,8 @@ public Task FileSnippetExplicitIncludeBypassesExcludeSnippetFiles()
         // `begin-snippet`/`end-snippet` markers, but an explicit `snippet: sourceFile.txt`
         // in a markdown file must still resolve to the whole-file contents — that lookup
         // goes through allFiles, which remains unfiltered.
-        var root = Path.GetFullPath("DirectoryMarkdownProcessor/FileSnippet");
+        // Own copy of the FileSnippet fixture, so it does not race FileSnippet writing the same one.md
+        var root = Path.GetFullPath("DirectoryMarkdownProcessor/FileSnippetExplicitInclude");
         var processor = new DirectoryMarkdownProcessor(
             root,
             writeHeader: false,