[yyakhnytsia]

Why

Slack's chat.update keeps a field when it's omitted and clears it only when sent as []. Since ChatUpdateMessageParams is @JsonInclude(NON_EMPTY), an empty attachments/blocks list is dropped — so there's currently no way to clear attachments or blocks via the typed client. This blocks updating a message from one representation to another (e.g. legacy attachments → Block Kit blocks): the old field lingers and the message renders both stacked.

What changed

• ChatUpdateMessageParamsIF — two @JsonIgnore @Default false flags, shouldClearAttachments() / shouldClearBlocks(). When set, the field serializes as []; default-off means serialization is unchanged for all existing callers.
• ForceEmitPropertyWriter (new) — BeanPropertyWriter that force-emits an empty field when the flag is set, else defers to the default writer.
• ChatUpdateMessageParamsSerializerModifier (new) — BeanSerializerModifier that swaps only the attachments/blocks writers on ChatUpdateMessageParams; all other fields keep their default writers.
• ObjectMapperUtils — registers the modifier on the shared mapper (early-returns for every other type).

Testing

ChatUpdateMessageParamsSerializationTest: 13 tests (omit-when-off, emit-[]-when-on, non-empty preserved, flag independence, flags don't leak, drift guard, other-types-untouched). Full slack-base suite: 369/369 pass.

[Viktor-Pelepiak]

LGTM