fix: duplicate message - WPB-11435

[netbe]

WPB-11435 [iOS] All messages in a 1:1 conversation duplicated in iOS

Issue

On the legacy code, it happens message get processed multiple times. It could be that in the following recursive method, because of a suspension point, the fetchNextEventsBatch fetch the same event before we actually delete it from the database and so reprocess it.

  private func process(
        with privateKeys: EARPrivateKeys?,
        _ consumeBlock: ConsumeBlock,
        firstCall: Bool,
        callEventsOnly: Bool

• Make EventDecoder an actor so it's always executed on the same queue.
• Safe guard the method with isProcessing so we don't process twice the same event.

Testing

1. on a conversation, run web spam messages
2. on the receiver side, send messages

Actual: You'll see duplicate messages

Fix: You'll get no duplicate messages

---

Checklist

• [ ] Title contains a reference JIRA issue number like [WPB-XXX].
• [ ] Description is filled and free of optional paragraphs.
• [ ] Adds/updates automated tests.

---

UI accessibility checklist

If your PR includes UI changes, please utilize this checklist:

• [ ] Make sure you use the API for UI elements that support large fonts.
• [ ] All colors are taken from WireDesign.ColorTheme or constructed using WireDesign.BaseColorPalette.
• [ ] New UI elements have Accessibility strings for VoiceOver.