IAV-989/feat(instagram): introduce dedicated Instagram Pro validation service with header-based token transport

Extracts Instagram credential validation out of the shared Meta store into a dedicated service layer.

What changed

New files:

• instagram.pro.validation.types.ts — types mirroring backend Java DTOs
• instagram-pro-api-validation-api-service.config.ts — endpoint constants
• instagram-pro-api-validation.service.api.ts — token sent via X-Instagram-Access-Token header
• instagram.pro.validation.store.ts — useInstagramProValidationStore with validateTokenPermissions, validateWebhook, resetValidations
• InstagramAccessTokenField.tsx — isolated token input with show/hide toggle

Modified:

• step-integration-config-dynamic.tsx — wires new store for instagram_header_auth; renders InstagramAccessTokenField instead of DynamicCredentialForm

Related Issues

Closes IAV-989