notification-service: split configureRouting to cut cognitive complexity (kotlin:S3776)

[devin-ai-integration]

Summary

Fixes SonarCloud kotlin:S3776 (issue_key AZ6OP-RWRoUIIbwnU9es, Cognitive Complexity 46 > 15) on Application.configureRouting in services/notification-service/.../routes/Routes.kt.

Behavior-preserving refactor: the single routing lambda is split into private Route extensions plus shared ApplicationCall helpers that replace the six copy-pasted user-id/notification-id guards:

fun Application.configureRouting(registry) = routing {
    get("/health") {...}; get("/metrics") {...}
    notificationRoutes(notificationService)      // /api/v1/notifications/**
    preferenceRoutes(notificationService)        // /api/v1/preferences
    notificationWebSocket(webSocketManager)      // /ws/notifications/{userId} -> handleIncomingFrames()
}

private fun ApplicationCall.requestedUserId(): String?          // X-User-ID header ?: user_id query, null if blank
private suspend fun ApplicationCall.respondUserIdRequired()      // 400, same message as before
private suspend fun ApplicationCall.respondNotificationIdRequired()
private suspend fun ApplicationCall.respondNoContentOrNotFound(success: Boolean)

Status codes, error messages and response bodies are unchanged. No rule suppression, no sonar config changes.

Testing

Added routes/RoutesTest.kt (ktor testApplication + Koin + mockk) covering the 400 missing-user-id path with exact body, unread-count with X-User-ID, mark-as-read 204/404, and /health. ./gradlew test and ./gradlew build pass locally in services/notification-service.

Link to Devin session: https://partner-workshops.devinenterprise.com/sessions/8a04b6f2804e42caa0c6ebdffd12f40d
Open in Devin Desktop: https://partner-workshops.devinenterprise.com/desktop/session/8a04b6f2804e42caa0c6ebdffd12f40d?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.