fix(relay-test-utils): fix plural enum field generation

[sjwilczynski]

Summary

• Fixed MockPayloadGenerator to return all enum values for plural enum fields instead of just the first value
• Added test to validate default enum array generation behavior with @relay_test_operation

Before: Plural enum fields like traits: [PersonalityTraits] would return ["<mock-value-for-field-\"traits\">"] (mock array, not matching the enum type)

After: Plural enum fields return all enum values: ["CHEERFUL", "DERISIVE", "HELPFUL", "SNARKY"]

Test plan

• Added new test generate mock for enum in arrays with default values that validates plural enum fields return all enum values by default
• Other tests still pass