[CHA-3618] feat: support custom_set and custom_unset in batch channel update - #270

[Must Fix] data plus customSet in one request always comes back 400.

ChannelDataUpdate has no null-inclusion setting, so Jackson writes every unset field, custom included:

{"operation":"updateData","filter":{...},"members":null,
 "data":{"frozen":true,"disabled":null,"custom":null,"team":null,...},
 "custom_set":{"group":"old"},"custom_unset":["location_id"]}

On the v1 route data.custom is the extra-fields sink, and a null-valued key is still captured into it (kit/jsonextra/decode_test.go:870, spec section 5.3). So Data.Custom decodes to ExtraFields{"custom": nil}, which is non-nil, and ChannelBatchUpdateRequest.ValidateStruct (chat/lib/chat/controller/v1/payload/channel_batch_update.go:57) returns:

'custom' (and any other custom field sent directly in 'data') replaces the whole custom object and thus cannot be used together with 'custom_set' or 'custom_unset'

That is the second example in the PR description and in docs/channels/channel_management/batch-updates.md. The custom-only path is fine, only the combined one fails.

Same root cause, already shipping today on the existing updateData(filter, data) path: "custom": null makes HasCustomUpdate() true, so chat/monolith/tasks/channelstasks/batch/operations/update_channel_data_processor.go:59-63 writes the custom column and replaces each matched channel's whole custom object with {"custom": null}. The channel display name lives in custom, so a batch updateData that only sets frozen deletes it.

Fix: @JsonInclude(JsonInclude.Include.NON_NULL) on ChannelDataUpdate.custom (Channel.java:2056), or class-level on ChannelDataUpdate to match ChannelsBatchFilters at Channel.java:2079.

data + patch  : {"operation":"updateData","filter":{"cids":{"$in":["messaging:a"]}},"members":null,"data":{"frozen":true,"disabled":null,"custom":null,"team":null,"config_overrides":null,"auto_translation_enabled":null,"auto_translation_language":null},"custom_set":{"group":"old"},"custom_unset":["location_id"]}
patch only    : {"operation":"updateData","filter":{"cids":{"$in":["messaging:a"]}},"members":null,"data":null,"custom_set":{"group":"old"}}

java-shape:     Custom==nil=false  Custom=jsonextra.ExtraFields{"custom":interface {}(nil)}
java-shape:     ValidateStruct=400 'custom' ... cannot be used together with 'custom_set' or 'custom_unset'
omitted-shape:  Custom==nil=true   ValidateStruct=<nil>
custom-only:    Custom==nil=true   ValidateStruct=<nil>

Fixed in 40194859 — ChannelDataUpdate.custom is now omitted when null, covering both the combined patch and existing data-only paths.

[Should Fix] These tests only look at the root field names, and that is the gap that let the data.custom 400 through: nothing here asserts what the serialized data object contains.

This line also pins the exact ordered list of all six root fields, so any future field added to ChannelsBatchOptions fails a test about the custom patch. Assert the absence of what could actually leak instead:

Assertions.assertFalse(root.has("customSet"));
Assertions.assertFalse(root.has("customUnset"));
Assertions.assertFalse(root.has("update"));

and add one case that serializes data and asserts custom is absent from it, which is the field the backend's mutual-exclusion check reads.

Fixed in 40194859 — the tests now assert that data.custom is absent for both paths and use targeted field-name assertions instead of an ordered root-field list.

[Should Fix] This also changes behavior for plain updateData calls that never touch custom: before this line the SDK always sent "custom": null, which replaced each matched channel's whole custom object. Please call that out in the release notes so upgrading users know their earlier batch updateData calls were affected.

Added a Release note section to the PR description covering the impact on existing updateData calls.