[Telemetry 1/4] Add telemetry hooks to Neo core

[Jim8y]

Summary

This PR adds static event hooks to the Neo core P2P layer to enable telemetry collection without modifying the core behavior.

Changes

• Add MessageSent event to RemoteNode for tracking outgoing messages
• Add ConnectionChanged event for monitoring peer connection lifecycle
• Wrap event invocations in try-catch to prevent telemetry failures from crashing the node

Files Changed

• src/Neo/Network/P2P/RemoteNode.cs

Test Plan

• [x] Build passes
• [x] Existing unit tests pass
• [x] Event handlers with exceptions don't crash the node

Part of Telemetry Feature

This is PR 1 of 4 for adding comprehensive telemetry support to Neo nodes:

1. [This PR] Core hooks - Add event hooks to Neo core
2. Plugin + Collectors - Main telemetry plugin and metric collectors
3. Health endpoints - Kubernetes-compatible health check endpoints
4. Documentation - README and monitoring configurations

[Jim8y]

As is suggested and requested from @shargon and @vncoelho , telemetry is split into 4 prs. These prs will not be merged into master directly, but will go to telemetry first.