Add tests for sendNtfy, sendSlack, SendNotification, newRequest,
ntfyPriority, and slackColor covering:
- Correct ntfy headers (Title, Priority) and body content
- All priority-to-ntfy-priority mappings
- Correct Slack/Mattermost JSON payload structure and Content-Type
- All priority-to-color mappings
- HTTP error handling (4xx, 5xx status codes)
- Network transport failures
- Goroutine dispatch to all configured endpoints
- Individual endpoint dispatch (ntfy-only, slack-only, mattermost-only)
- No-op behavior when no webhooks are configured
- Error logging paths in SendNotification goroutines
- SlackPayload JSON marshaling and omitempty behavior
- HTTP request construction via newRequest
- Context propagation through newRequest
Uses httptest.Server for HTTP endpoint testing and a custom
failingTransport for network error simulation. Test helpers are
exported via export_test.go following standard Go patterns.
Coverage improved from 11.1% to 80.0% for the notify package.
