fix(settings): preserve Apple additional client IDs state

[pjpjq]

Summary

• preserve Apple auth client ID fields when external_apple_additional_client_ids is configured
• avoid API read/write responses that fold additional Apple IDs into external_apple_client_id from changing Terraform state after apply
• add a regression test for the merged API response shape

Fixes #336

Tests

• go test ./internal/provider -run 'TestAccSettingsResource_AppleAdditionalClientIds|TestParseConfigNullablePreservesUserValue' -count=1
• go test ./...
• test -z "$(gofmt -l internal/provider/settings_resource.go internal/provider/settings_resource_test.go)"
• git diff --check