fix(settings): preserve Apple additional client IDs state

[pjpjq]

Replaces #340, which was closed when the fork branch was renamed from codex/... to fix/....

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

[JEETDESAI25]

Hey @pjpjq, thanks for the fix and regression test.

This looks good to me. The helper matches the existing preserved-from-state pattern, and the test covers the lossy API response from issue #336 where additional Apple IDs are folded into external_apple_client_id.

No blockers from me @savme