fix(audience): treat missing queue dir as empty queue in ReadBatch (SDK-341)

- DiskStore.ReadBatch was the one path that called
  Directory.GetFiles(_queueDir) without the DirectoryNotFoundException
  guard the rest of the methods (DeleteAll, ApplyAnonymousDowngrade,
  TryDelete) already use.
- Linux Mono / IL2CPP test cells run a SetUp that deletes the SDK
  persistent dir between every test. A background flush timer started
  by the prior test can fire after the delete; the resulting GetFiles
  raised DirectoryNotFoundException which propagated through
  HttpTransport.SendBatchAsync to OnError, made the SampleApp log
  "flush() Err" instead of "flush() Ok", and failed the test.
- Same shape was hitting Shutdown's flush path: the AggregateException
  the caller saw was wrapping the same DirectoryNotFoundException.
- Catch + return Array.Empty<string>(). Empty result is the correct
  semantics: a deleted queue dir has no events to send.

Surfaced by run 25539153233 once SDK-317 (PR #754) landed real Linux
PlayMode coverage. Affected 9 cells (2 Mono, 7 IL2CPP) on Unity 6.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ImmutableJeffrey

src/Packages/Audience/Runtime/Transport/DiskStore.cs

@@ -65,9 +65,18 @@ internal IReadOnlyList<string> ReadBatch(int maxSize)
 
             var result = new List<string>();
 
-            // Sort by filename (ticks prefix) → oldest first
-            var files = Directory.GetFiles(_queueDir, "*.json")
-                .OrderBy(f => Path.GetFileName(f), StringComparer.Ordinal);
+            // Sort by filename (ticks prefix) → oldest first.
+            // The queue dir can disappear out from under us when an external
+            // process (test SetUp, manual cleanup, persistentDataPath wipe)
+            // deletes it between SDK Init and the next flush tick. Treat that
+            // as an empty queue rather than letting DirectoryNotFoundException
+            // bubble up through SendBatchAsync to the caller's OnError.
+            // Matches the existing guard in DeleteAll / ApplyAnonymousDowngrade.
+            string[] paths;
+            try { paths = Directory.GetFiles(_queueDir, "*.json"); }
+            catch (DirectoryNotFoundException) { return Array.Empty<string>(); }
+
+            var files = paths.OrderBy(f => Path.GetFileName(f), StringComparer.Ordinal);
 
             foreach (var path in files)
             {