IAV-1203/feat(wizard): fix webhook URLs, wizard steps tracking, WhatsApp OAuth integration & N8N response parsing

Description

Bug fixes and improvements across the Agent Wizard flow, WhatsApp integration, and N8N webhook communication.

Bug Fixes

• Fix N8N webhook URL double-slash (wrong N8N_PREFIX_WEBHOOK_PATH constant)
• Fix missing wizard step updates (CONTEXT, IDENTITY, INTEGRATIONS)
• Fix WhatsApp credential resolution (merge primary + OAuth secondary before platform ID check)
• Fix N8nWebhookResponse field mismatch (output → message)

Improvements

• Expose updateWizardStep in common service interface
• Add wizardCompleted to sub-agent list response
• Improve N8N response parsing (array, object, plain text)
• Add WhatsApp OAuth credential mapper and merge utilities

Related Issues

Closes IAV-1203