Add discriminated union serialization tests

[jar-stripe]

Why?

Validates that the discriminated union serialization design works correctly in stripe-java before codegen produces these patterns. Exercises both request-side (ApiRequestParamsConverter serialization) and response-side (Gson polymorphic deserialization via TypeAdapterFactory).

What?

• Adds DiscriminatedUnionSerializationTest with 7 tests covering:
  • Request standalone union: Object-typed field on parent params holding variant class instances, each with a fixed discriminator initializer. Verifies ApiRequestParamsConverter.convert() produces correct nested map structure.
  • Request inline union: discriminator as String field + typed variant params fields on parent. Verifies non-selected variant is null in output.
  • Response standalone union: Base class (TestColorEntity) with only discriminator field + variant subclasses extending it. Custom TestColorTypeAdapterFactory dispatches on discriminator value to correct subclass. Verifies instanceof and typed field access.
  • Response inline union: flat class with discriminator + nullable variant entity fields. Verifies correct Gson deserialization.

[Copilot]

Pull request overview

Adds a new unit test suite to validate the “discriminated union” serialization/deserialization patterns used in stripe-java, covering request-side params-to-map conversion via ApiRequestParamsConverter and response-side JSON-to-model parsing via ApiResource.GSON.

Changes:

• Introduces DiscriminatedUnionSerializationTest with request-side coverage for standalone (object-typed) and inline (parent-level discriminator + nested typed params) union shapes.
• Adds response-side deserialization coverage for the same standalone and inline union shapes.

Suppressed comments (2)

src/test/java/com/stripe/net/DiscriminatedUnionSerializationTest.java:237

• The inline-union request serialization test doesn't assert that the non-selected variant field is omitted. Adding an explicit check helps ensure only the chosen variant is serialized.

    Map<String, Object> map = converter.convert(params);
    assertEquals("bank", map.get("type"));

    Map<String, Object> bankMap = (Map<String, Object>) map.get("bank");
    assertEquals("110000000", bankMap.get("routing_number"));
    assertEquals("000123456789", bankMap.get("account_number"));

src/test/java/com/stripe/net/DiscriminatedUnionSerializationTest.java:267

• The inline-union response deserialization test should also assert that the non-selected variant (bank) is null. Otherwise, a regression that populates both variants would still pass.

    assertEquals("card", entity.type);
    assertEquals("4242424242424242", entity.card.number);
    assertEquals(Long.valueOf(12L), entity.card.expMonth);

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[jar-stripe]

Added — the RGB deserialization test now asserts h/s/v are null, and the card deserialization test asserts bank is null.

[jar-stripe]

Added — the card serialization test now asserts that bank is absent from the output map.