Complete unit test coverage for remaining core modules

[georgi-l95]

Description:

Add comprehensive unit tests for remaining rock-node-core modules:

• block_reader.rs / block_writer.rs - Test block I/O operations
• app_context.rs - Test context initialization and service providers
• events.rs - Test event types and handling
• error.rs - Test error conversions and formatting
• cache.rs - Test caching behavior

Acceptance Criteria:

• 80%+ test coverage for rock-node-core crate
• All edge cases and error scenarios covered
• Enhanced test utilities for reuse