fix: address race condition in continuation-based waiting

Add recheck of message count after acquiring lock and before storing
continuation to prevent race where message arrives between initial
count check and continuation install.

Addresses Codex review feedback on PR #931.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

bitchatTests/Fragmentation/FragmentationTests.swift

@@ -263,6 +263,13 @@ extension FragmentationTests {
                 group.addTask {
                     await withCheckedContinuation { continuation in
                         self.lock.lock()
+                        // Recheck count after acquiring lock to avoid race condition
+                        // where message arrives between initial check and continuation install
+                        if self._publicMessages.count >= count {
+                            self.lock.unlock()
+                            continuation.resume()
+                            return
+                        }
                         self.publicMessageContinuation = continuation
                         self.lock.unlock()
                     }
@@ -290,6 +297,13 @@ extension FragmentationTests {
                 group.addTask {
                     await withCheckedContinuation { continuation in
                         self.lock.lock()
+                        // Recheck count after acquiring lock to avoid race condition
+                        // where message arrives between initial check and continuation install
+                        if self._receivedMessages.count >= count {
+                            self.lock.unlock()
+                            continuation.resume()
+                            return
+                        }
                         self.receivedMessageContinuation = continuation
                         self.lock.unlock()
                     }