Test/add service unit tests

[Alaka-ibr]

Completed Work

1. Closes #699: Escalation Service ✅

File: backend/tests/services/escalation.service.test.ts
Lines: 581
Test Coverage:
Incident creation with various severity levels
Incident acknowledgement and resolution
Escalation logic with threshold handling
Escalation rule management
Timeout-based escalation
Acknowledgement requirement handling
Engine start/stop functionality
History tracking
Edge cases and error handling

2. Closes #700: Digest Scheduler Service ✅

File: backend/tests/services/digestScheduler.service.test.ts
Lines: 746
Test Coverage:
Subscription creation and management
Daily and weekly digest generation
Quiet hours logic and timezone handling
Delivery scheduling and processing
Email notification integration
Retry mechanisms for failed deliveries
History retrieval and unread counts
Filter and preference handling

3. Closes #701: Dependency Graph Service ✅

File: backend/tests/services/dependencyGraph.service.test.ts
Lines: 412
Test Coverage:
Graph retrieval with all nodes and edges
Node type filtering (bridge, oracle, indexer, rpc, queue, database, notifier, api)
Status filtering (healthy, degraded, down, unknown)
Search functionality
Summary statistics calculation
Edge relationship validation
Multiple filter combinations
Case-insensitive and trimmed input handling

4. Closes #702: External Dependency Monitor Service ✅

File: backend/tests/services/externalDependencyMonitor.service.test.ts
Lines: 701
Test Coverage:
Dependency listing with history
Health check execution (HTTP and JSON-RPC)
Status determination logic
Maintenance mode handling
Alert threshold triggering
Consecutive failure tracking
Timeout handling
Different check types for various providers

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