test: Improve pkg/exporters/http test coverage from 75.6% to 88.3%

Add comprehensive tests for HTTP exporter functionality including:
- IsReloadable and Reload methods with various config scenarios
- needsWorkerPoolRecreation logic for detecting config changes
- performHealthCheck for webhook health monitoring
- min helper function edge cases

Create new stats_test.go with tests for all stats functions:
- RecordDroppedRequest and RecordRetryAttempt tracking
- GetTotalFailures, GetStatusSuccessRate, GetProblemSuccessRate
- WebhookStatsSnapshot GetSuccessRate and IsHealthy methods
- recordRequest with response time averaging
- Thread-safe concurrent access validation

TaskForge: #3831

Author: mattmattox