#25120 fix(java): serialize jersey multipart object parts as JSON and honor …

Closes #25119

@bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10) @martin-mfg (2023/08) @KannaKim (2026/07)

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Summary by cubic

Fixes Jersey multipart/form-data serialization so complex parts (objects, arrays) are sent as JSON instead of plain strings, and per-part contentType declared in the spec's encoding is honored.

• serialize gains an overload accepting per-part content types; the existing signature delegates with an empty map, so callers are unaffected.
• Generated API templates populate content types from the request body encoding declarations.
• Adds tests covering JSON-serialized complex parts and encoding-based content types, including a new multipart-encoding.yaml spec.