Summary

• Adds a parameterized test (GraphqlDecoderOptionalFieldCodecTest) that verifies Optional<T> record fields are correctly decoded by the GraphQL decoder using MockClient
• Tests 3 payload scenarios: explicit null ("step": null), missing field, and present value — across Jackson and Jackson3 codecs
• Adds feign-mock and feign-jackson3 as test dependencies to the graphql module

Test plan

• All 6 parameterized test cases pass (3 payloads × 2 codecs)
• Full graphql module test suite passes (51 tests)

Gson, Moshi, and Fastjson2 were evaluated but excluded — they don't support Optional<T> fields in records.

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