SYN-3601: Preserve certificates with nullable HTTP ports

[jcsco]

Summary

Preserves HTTP V2 client-certificate support when callers use the existing nullable-port request and response models.

Problem

HttpCheckV2Input and HttpCheckV2Response include certificateId, while HttpCheckV2InputWithNullablePort and HttpCheckV2ResponseWithNullablePort do not. The Terraform provider must use the nullable-port models to represent an omitted HTTP port separately from an explicit 0. Without this change, adopting those models for SYN-3601 would silently omit or lose the HTTP client-certificate association added by SYN-5402.

Changes

• Add certificateId to HttpCheckV2InputWithNullablePort.
• Add certificateId to HttpCheckV2ResponseWithNullablePort.
• Verify create serializes a configured certificate ID.
• Verify update serializes explicit null for certificate removal.
• Verify get parses the certificate ID alongside nullable port values.

Compatibility

This is additive and preserves the existing JSON field name and nullable semantics. Existing callers that do not set a certificate ID are unchanged.

Validation

• go test ./... -tags=unit_tests -count=1 -timeout=60s
• go vet -tags=unit_tests ./...
• go build -tags=unit_tests ./...
• golangci-lint run ./syntheticsclientv2/...
• git diff --check

Related

• SYN-3601
• terraform-provider-synthetics#90
• terraform-provider-synthetics#97