[automatic failover] Record failures for each attempt of write on channel including retries

[atakavci]

Introduce new channel handler and extract command tracking logic and add Netty pipeline handler

Summary

This PR refactors the failover module to separate command tracking concerns and introduces a Netty pipeline handler for comprehensive command result tracking.

Changes

1. New DatabaseCommandTracker class

• Extracts command tracking logic from DatabaseEndpointImpl and DatabasePubSubEndpointImpl
• Implements CommandWriter interface for delegation pattern
• Manages circuit breaker binding and channel lifecycle
• Tracks timeout exceptions via onComplete() callbacks
• Records exceptions thrown during write operations
• Handles both single and batch command writes

2. New MultiDbOutboundAdapter Netty handler

• Intercepts write operations in the Netty pipeline
• Tracks non-timeout exceptions and successes via command completion callbacks
• Complements DatabaseCommandTracker for comprehensive result tracking
• Registered after CommandHandler in the pipeline

3. Refactored DatabaseEndpointImpl and DatabasePubSubEndpointImpl

• Simplified by delegating to DatabaseCommandTracker
• Implements CommandWriter interface
• Manages channel lifecycle (notifyChannelActive/notifyChannelInactive)
• Reduced code duplication (~60 lines removed from each class)

Architecture

The new architecture splits command result tracking into two layers:

1. DatabaseCommandTracker (callback-based):

   • Tracks timeout exceptions via onComplete() callbacks
   • Records exceptions thrown during write operations
   • Manages circuit breaker state checks

2. MultiDbOutboundAdapter (pipeline-based):

   • Tracks write failures in the Netty pipeline
   • Tracks non-timeout exceptions and successes via command completion
   • Provides comprehensive coverage of all command results

Tests

• New: DatabaseCommandTrackerUnitTests (527 lines) - comprehensive unit tests
• New: MultiDbOutboundAdapterUnitTests (309 lines) - unit tests for pipeline handler
• New: DatabasePubSubEndpointTrackerTests (310 lines) - PubSub-specific tests
• Updated: DatabaseEndpointCallbackTests - refactored to focus on integration behavior and avoid duplication

All tests passing: 44 unit tests covering the new architecture

Benefits

• ✅ Separation of concerns (tracking logic extracted from endpoints)
• ✅ Reduced code duplication
• ✅ Comprehensive command result tracking (both callback and pipeline-based)
• ✅ Better testability with focused unit tests
• ✅ Maintains backward compatibility