test: add comprehensive tests for all Epic 1 stories

Story 1.2: Database Layer
- Test database client creation, connection, ping, and close
- Test connection pooling configuration
- Tests skip if database is not available (short mode)

Story 1.3: Health Monitoring and Metrics
- Test health registry registration and checking
- Test database health checker
- Test liveness and readiness checks
- Test metrics creation, middleware, and handler
- Test Prometheus metrics endpoint

Story 1.4: Error Handling and Error Bus
- Test channel-based error bus creation
- Test error publishing with context
- Test nil error handling
- Test channel full scenario
- Test graceful shutdown
- Fix Close() method to handle multiple calls safely

Story 1.5: HTTP Server and Middleware
- Test server creation with all middleware
- Test request ID middleware
- Test logging middleware
- Test panic recovery middleware
- Test CORS middleware
- Test timeout middleware
- Test health and metrics endpoints
- Test server shutdown

Story 1.6: OpenTelemetry Tracing
- Test tracer initialization (enabled/disabled)
- Test development and production modes
- Test OTLP exporter configuration
- Test graceful shutdown
- Test no-op tracer provider

All tests follow Go testing best practices:
- Table-driven tests where appropriate
- Parallel execution
- Proper mocking of interfaces
- Skip tests requiring external dependencies in short mode

internal/errorbus/channel_bus.go

@@ -12,11 +12,12 @@ import (
 
 // ChannelBus implements a channel-based error bus.
 type ChannelBus struct {
-	errors chan errorWithContext
-	logger logger.Logger
-	done   chan struct{}
-	wg     sync.WaitGroup
-	once   sync.Once
+	errors    chan errorWithContext
+	logger    logger.Logger
+	done      chan struct{}
+	wg        sync.WaitGroup
+	once      sync.Once
+	closeOnce sync.Once
 }
 
 type errorWithContext struct {
@@ -157,7 +158,9 @@ func (b *ChannelBus) Close() error {
 		close(b.done)
 	})
 	b.wg.Wait()
-	close(b.errors)
+	b.closeOnce.Do(func() {
+		close(b.errors)
+	})
 	return nil
 }