Add component tests: Notification channels, rules & processing

Add component tests for notification system.

Target test file: tests/component/notification/test_notification_full.py

Scenario files: notification/channel_management.md (19), notification/rule_management.md (24), notification/notification_processing.md (15) = 58 total

Success criteria:
- test file created and all tests pass
- Webhook and Email channel CRUD tested
- Channel validation (test message send) tested
- Rule CRUD with type list/schema and template validation tested
- Event-driven notification processing with partial failure tolerance tested
- Permission: only admin can manage channels/rules (403)

JIRA Issue: BA-5005