fix(ChatViewModel): add deinit cleanup for NotificationCenter observers and timers

[samyakjain-1]

fix(ChatViewModel): add deinit cleanup for NotificationCenter observers and timers

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Summary

Added a deinit method to ChatViewModel to clean up NotificationCenter observers, timers, and Combine subscriptions when the view model is deallocated. Without this, resources would persist beyond the lifecycle of the instance, leading to memory leaks and unpredictable behavior.

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Problem

ChatViewModel registers for system notifications during initialization using:

NotificationCenter.default.addObserver(self, selector: ..., name: ..., object: nil)

It also creates:

• A debounce timer (nicknameSaveTimer) in the nickname setter
• A Combine subscription (deliveryTrackerCancellable) to track delivery status

Previously, there was no deinit method, so none of these were cleaned up when the object was released. This could lead to:

• Retained memory (due to observers)
• Dangling callbacks on deallocated instances
• Flaky behavior during testing or navigation

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Fix

Added a deinit method to the class (within the primary ChatViewModel declaration):

deinit {
    NotificationCenter.default.removeObserver(self)
    nicknameSaveTimer?.invalidate()
    deliveryTrackerCancellable?.cancel()
}

This ensures proper cleanup when the view model is deallocated.

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This:

• Prevents memory leaks: by removing retained observers
• Improves test reliability: by preventing callbacks on dead objects
• Brings lifecycle hygiene: in line with best practices used across the codebase