IAV-1273/feat: delete source AgentIntegration after successful replication for channel-type credentials

Summary

Channel-type integrations (Telegram, WhatsApp, Facebook Messenger, Instagram Header Auth) can only be active in one N8N instance at a time. Previously, replication would create a new AgentIntegration without removing the old one, leaving orphaned integrations in the database.

Changes

• After a successful replication, the source AgentIntegration is now deleted only for channel-type credential types (NON_REPLICABLE_CREDENTIAL_TYPES)
• Secondary credentials of the source integration are deleted first to respect FK constraints
• Non-channel integrations (SLACK, GMAIL, etc.) are unaffected — their source is preserved
• Added a null check on the source integration DTO before the connectivity check, raising NO_SOURCE_INTEGRATION_FOUND instead of an NPE
• Updated the replication blocked error message to clearly explain that the integration must be disconnected (connected=false) before replication is allowed

Transactional Safety

All operations (create new + delete old) run inside the same @Transactional boundary.

Scenario	Behavior
Replication succeeds	New integration created ✅ + Source deleted ✅
Failure during credential replication	Full rollback — source preserved ✅
Failure during new integration save	Full rollback — source preserved ✅
Failure during source deletion	Full rollback — neither persisted ✅

Scope

Credential type	Source deleted after replication
TELEGRAM, WHATSAPP, FACEBOOK_MESSENGER, INSTAGRAM_HEADER_AUTH	✅ Yes
All other types	❌ No