🌱 Replace with comprehensive tests for stellar/notifications.go

Replaces incomplete tests with comprehensive coverage for all 10 functions:

HTTP Handlers:
- ListNotifications: empty result, multiple notifications, unread filter
- MarkNotificationRead: success, missing ID validation  
- MarkNotificationInvestigating: success with note, invalid JSON
- ResolveNotification: success with metadata population
- DismissNotification: success with state transition
- updateNotificationState: not found, wrong user, AffectedResource derivation

Helper Functions:
- describeNotificationStateChange: all statuses (investigating, resolved, dismissed, unknown) with/without notes
- deriveNotificationWorkload: ev prefix, no prefix, edge cases
- deriveStellarNotificationResource: standard format, fallbacks

All tests follow stellar package patterns: Fiber test helpers, testify assertions, SQLiteStore integration, user isolation

Signed-off-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: clubanderson