Add recommended tests and suggested enhancements for MarketDepthManager

Author: codez0mb1e

docs/MarketDepthManager_Improvements.md

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+# `MarketDepthManager` Improvements
+
+## 📋 Recommended Additional Tests
+
+### Integration-Style Tests (High Priority)
+
+```csharp
+public class MarketDepthManagerIntegrationTests
+{
+    [Test]
+    public async Task BuildAsync_WithRealTimeSimulation_SynchronizesCorrectly()
+    {
+        // Test that simulates realistic WebSocket + REST timing
+        // Validates the 7-step algorithm with controlled event sequences
+    }
+
+    [Test]
+    public async Task BuildAsync_WithSlowSnapshot_HandlesBufferedEvents()
+    {
+        // Simulates slow snapshot retrieval while events keep arriving
+        // Ensures buffer doesn't overflow and events are properly queued
+    }
+
+    [Test]
+    public async Task BuildAsync_WithOutdatedSnapshot_RetriesUntilValid()
+    {
+        // Tests snapshot validation and retry logic (Step 4)
+        // Ensures system gets a fresh enough snapshot
+    }
+
+    [Theory]
+    [InlineData(ContractType.Spot)]
+    [InlineData(ContractType.Futures)]
+    public async Task ProcessDepthUpdate_WithSequentialEvents_MaintainsOrderBook(ContractType contractType)
+    {
+        // Validates that sequential updates are processed correctly
+        // Tests both Spot and Futures contract types
+    }
+}
+```
+
+### Edge Case Tests (Medium Priority)
+
+```csharp
+[Test]
+public async Task BuildAsync_WithNoBufferedEvents_StillAppliesSnapshot()
+{
+    // Edge case: snapshot arrives before any WebSocket events
+}
+
+[Test]
+public async Task ProcessDepthUpdate_WithDuplicateUpdateId_IgnoresCorrectly()
+{
+    // Validates idempotency - duplicate events should be ignored
+}
+
+[Test]
+public async Task BuildAsync_CancellationToken_CancelsGracefully()
+{
+    // Tests cancellation during various stages of BuildAsync
+}
+
+[Test]
+public async Task StopStreamingAsync_WithActiveSubscription_UnsubscribesCleanly()
+{
+    // Validates proper cleanup of WebSocket subscriptions
+}
+```
+
+---
+
+## 🔄 Suggested Future Enhancements
+
+### 1. Configuration Object Pattern
+Instead of multiple parameters, consider a configuration object:
+
+```csharp
+public class MarketDepthConfiguration
+{
+    public short OrderBookDepth { get; init; } = 10;
+    public TimeSpan UpdateInterval { get; init; } = TimeSpan.FromMilliseconds(100);
+    public TimeSpan BufferTimeMultiplier { get; init; } = TimeSpan.FromMilliseconds(5);
+    public int MinimumBufferTimeMs { get; init; } = 500;
+}
+
+public async Task BuildAsync(
+    MarketDepth marketDepth,
+    MarketDepthConfiguration config = null,
+    CancellationToken ct = default)
+```
+
+### 2. Resilience Improvements
+```csharp
+// Add automatic reconnection on WebSocket disconnect
+// Add configurable retry policy for snapshot retrieval
+// Add circuit breaker pattern for repeated failures
+```
+
+### 3. Observability Enhancements
+```csharp
+public class MarketDepthMetrics
+{
+    public long TotalUpdatesProcessed { get; set; }
+    public long UpdatesIgnored { get; set; }
+    public long SnapshotRetries { get; set; }
+    public TimeSpan AverageUpdateLatency { get; set; }
+}
+
+public MarketDepthMetrics GetMetrics() => _metrics;
+```
+
+### 4. Event-Driven Architecture
+```csharp
+public event EventHandler<SnapshotAppliedEventArgs> SnapshotApplied;
+public event EventHandler<SynchronizationErrorEventArgs> SynchronizationError;
+public event EventHandler<MetricsUpdatedEventArgs> MetricsUpdated;
+```
+