feat: implement network connectivity monitor interface and mock service (#26)

[rohansaini-02]

Summary

Implementation of a decoupled Connectivity Monitor Utility Proof of Concept (PoC). This module introduces an interface-driven architecture for monitoring network states, ensuring the core application remains agnostic of underlying native network libraries.

Related Issue

Resolves #26

Problem Statement

The TAP Buddy application relies on accurate network state detection for critical features like the Offline Sync Engine and AI Tutor WebSockets. Hardcoding a specific library (like NetInfo) directly into these services creates tight coupling and makes unit testing network transitions (e.g., WiFi to Offline) impossible without a physical device.

Changes Made

• Isolated Module Creation: Developed the poc-connectivity standalone TypeScript package.
• INetworkService Interface: Defined a strict contract for network monitoring, including synchronous state retrieval and an observer pattern for changes.
• MockNetworkService Implementation: Created a robust mock service with helper methods (simulateOffline(), simulateOnline()) for environment simulation.
• Observer Pattern: Implemented a listener subscription model to notify the application of connectivity changes in real-time.
• Automated Validation: Integrated a Jest test suite with 100% coverage across statements, branches, and functions.
• Housekeeping: Added a local .gitignore to keep the module structure clean and free of build artifacts.

Technical Decisions

• Interface Segregation: Chose an interface-driven approach to allow seamless swapping of the mock implementation with the real @react-native-community/netinfo later without modifying the Sync Engine.
• Mock-First Development: Prioritized a mockable implementation to unblock the functional validation of the Sync Engine ([PoW] Offline-First Sync Engine: Queue-Based Write Architecture #13) in local development environments.
• Zero-Dependency Architecture: The core logic is built with pure TypeScript to ensure maximum portability across the project.

Testing Performed

• 100% Code Coverage: Verified all logic paths using Jest.
• State Transition Testing: Confirmed isOnline() logic across various simulated states (WiFi, Cellular, None).
• Listener Validation: Verified that subscribers are correctly notified during state transitions and that the initial state is pushed upon subscription.
• Cleanup Logic: Confirmed that unsubscribe() correctly removes listeners to prevent memory leaks.

Results

A fully testable connectivity utility that provides the foundation for reliable offline-first syncing.

Checklist

• Code follows project conventions
• Tests pass (100% coverage achieved)
• Documentation updated (README included in PoC)
• Changes are scoped to one problem