notification-service: extract repeated route error messages into constants (kotlin:S1192)

[devin-ai-integration]

Summary

Fixes two SonarCloud kotlin:S1192 (duplicated string literal) findings in services/notification-service/.../routes/Routes.kt:

• AZ6OP-RWRoUIIbwnU9eq — "Notification ID is required" repeated 3× (GET/PUT/DELETE /{id} handlers)
• AZ6OP-RWRoUIIbwnU9er — "Notification not found" repeated 3× (same three handlers)

Both messages now live as top-level constants in Routes.kt and every call site references them, so the client-facing error text can only diverge in one place:

const val NOTIFICATION_ID_REQUIRED = "Notification ID is required"
const val NOTIFICATION_NOT_FOUND = "Notification not found"

Response bodies are byte-for-byte unchanged.

Test

New routes/RoutesTest.kt boots the routing module under Ktor's test host with a mocked NotificationService (via Koin) and asserts that GET /api/v1/notifications/{id}, PUT /{id}/read, and DELETE /{id} all return 404 with the shared NOTIFICATION_NOT_FOUND body, plus a guard that the constants carry the original messages. It does not compile against main (the constants don't exist there).

cd services/notification-service && ./gradlew build — 35 tests, all green.

Link to Devin session: https://partner-workshops.devinenterprise.com/sessions/295ba1f62fdf4469ba720d4b7bc81fe1
Open in Devin Desktop: https://partner-workshops.devinenterprise.com/desktop/session/295ba1f62fdf4469ba720d4b7bc81fe1?variant=devin
Requested by: @mbatchelor81

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✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.