IAV-841/feat(integrations): Meta credential validation for Facebook Messenger and Instagram

Summary

Adds real-time credential validation for Meta platforms (Facebook Messenger & Instagram). This includes token validation, permission analysis, webhook subscription verification, and page/account discovery via the Meta Graph API.

Motivation

Meta platforms require a more complex validation flow than other integrations:

• Tokens have types (User vs Page) and can expire
• Permissions must be checked individually against the Graph API
• Webhooks must be properly subscribed with the correct callback URL and fields

This MR ensures users get full visibility into their Meta integration health before creating workflows.

Changes

New Services

• MetaGraphApiService: Comprehensive Meta Graph API client handling token debug, permissions, page listing, Instagram account discovery, and webhook verification

New Endpoints

Method	Endpoint	Description
POST	/api/customer-care/integrations/meta/messenger/validate-token	Full Messenger validation (token + permissions + webhook)
POST	/api/customer-care/integrations/meta/instagram/validate-token	Full Instagram validation (token + permissions + webhook)
GET	/api/customer-care/integrations/meta/token/debug	Debug token info
GET	/api/customer-care/integrations/meta/token/permissions	Get granted permissions
GET	/api/customer-care/integrations/meta/facebook/pages	List user's Facebook pages
GET	/api/customer-care/integrations/meta/facebook/pages/{pageId}	Get page details
GET	/api/customer-care/integrations/meta/instagram/account	Find linked Instagram account
GET	/api/customer-care/integrations/meta/instagram/accounts/{accountId}	Get Instagram account details

Validation Flow

1. Token debug — call /debug_token to verify validity, type, and expiry
2. Permission check — call /me/permissions and compare against required list
3. Webhook verification — call /{page}/subscribed_apps to ensure correct callback URL, active status, and required subscription fields

Configuration

• MetaWebhookConfig: Webhook callback URLs, object types, and required fields for Messenger and Instagram

DTOs

• MetaTokenValidationResult: Comprehensive result with token info, permissions, and webhook status
• MetaDebugTokenResponse: Token introspection data
• MetaPermissionResponse: Granted/declined permissions
• MetaPageResponse, MetaPageListResponse: Facebook Page data
• MetaInstagramAccountResponse: Instagram Business Account data
• MetaWebhookSubscriptionResponse, MetaAppSubscriptionsResponse: Webhook subscription data

Error Codes (BusinessErrorCode 720–737)

• META_API_ERROR (720) — Generic Meta API error
• META_AUTHENTICATION_FAILED (721) — Auth failure
• META_INVALID_TOKEN (722) — Invalid token
• META_PERMISSION_DENIED (723) — Permission denied
• META_PAGE_NOT_FOUND (724) — Page not found
• META_INSTAGRAM_NOT_LINKED (725) — Instagram not linked
• META_WEBHOOK_NOT_CONFIGURED (726) — Webhook missing
• META_WEBHOOK_INACTIVE (727) — Webhook inactive
• META_WEBHOOK_URL_MISMATCH (728) — URL mismatch
• META_INVALID_TOKEN_TYPE (729) — Wrong token type
• META_WEBHOOK_URL_INVALID (730) — Invalid webhook URL
• META_TOKEN_EXPIRED (731) — Expired token
• META_ACCOUNT_NOT_FOUND (732) — Account not found
• META_INSUFFICIENT_PERMISSIONS (733) — Insufficient perms
• META_RATE_LIMIT_EXCEEDED (734) — Rate limited
• META_NETWORK_ERROR (735) — Network error
• META_WEBHOOK_MISSING_FIELDS (736) — Missing webhook fields
• META_RESOURCE_NOT_FOUND (737) — Resource not found

Testing

• Unit tests for MetaGraphApiService
• Integration tests with mocked Graph API
• Manual testing with real Meta tokens (staging)
• Verify webhook validation with live callback URLs

## Related Issues
Closes IAV-841