fix: Improve thread safety

[steve228uk]

Summary

• Adds an onMain helper to dispatch work to the main thread, executing synchronously if already on the main thread
  and asynchronously otherwise.
• Updates sendEventToJS in ExponeaBridge to dispatch through onMain, ensuring all in-app message delegate
  callbacks and segmentation callbacks reach the React Native bridge on the main thread.
• Updates notifyDimensChanged, notifyContentBlockCarouselEvent, notifyContentFilterRequest, and
  notifyContentSortRequest in CarouselInAppContentBlockViewProxy to dispatch through onMain.
• Updates notifyDimensChanged and notifyInAppContentBlockEvent in InAppContentBlocksPlaceholder to dispatch
  through onMain.

Issues Resolved

SDK delegate callbacks for in-app messages, content blocks, and segmentation arrive on background threads. Calling
RCTEventEmitter.sendEvent and UIKit layout APIs (dimension change notifications) from a background thread causes
undefined behaviour under the New Architecture / Fabric renderer and can result in crashes or dropped events.

How

• DictionaryExtensions.swift: Adds onMain(_:) — a free function matching the pattern used in the upstream
  exponea-ios-sdk. When the caller is already on the main thread it executes the block inline; otherwise it schedules
  it with DispatchQueue.main.async. This avoids unnecessary re-queuing for callbacks that already arrive on main.
• ExponeaBridge.swift: sendEventToJS wraps its eventEmitter?.sendEvent call in onMain, covering all event
  emission paths: inAppMessageShown, inAppMessageError, inAppMessageClickAction, inAppMessageCloseAction, and
  emitNewSegments.
• CarouselInAppContentBlockViewProxy.swift / InAppContentBlocksPlaceholder.swift: Each notify* method
  serialises any SDK reference types to value-safe representations (e.g. NSDictionary) on the calling thread before
  the onMain hop, so no SDK objects cross a thread boundary.