feat(model): add carousel and card Block Kit blocks

[zimeg]

Summary

Adds the carousel layout block and its card element to the slack-api-model block model, following the existing block conventions (Lombok @Data @Builder, TYPE constant, factory registration, DSL helper, round-trip test).

• CarouselBlock (type carousel): elements (1-10 CardBlock), blockId
• CardBlock (type card): heroImage, icon, slackIcon, title, subtitle, body, subtext, actions (an ActionsBlock, max 3 buttons), blockId. At least one of heroImage/title/actions/body must be present.
• Registered both in GsonLayoutBlockFactory
• Added Blocks.carousel(...) and Blocks.card(...) DSL helpers
• Added parseCarouselBlock and buildCarouselBlock (round-trip) tests in BlockKitTest

The card block is included because carousel.elements[] is an array of card blocks — the carousel cannot be modeled or tested without it. Both are net-new to the Java SDK.

Docs

• Carousel: https://docs.slack.dev/reference/block-kit/blocks/carousel-block
• Card: https://docs.slack.dev/reference/block-kit/blocks/card-block

Validation

Ran the touched package's tests:

./mvnw -pl slack-api-model test -Dtest=BlockKitTest -Djacoco.skip=true
=> Tests run: 45, Failures: 0, Errors: 0, Skipped: 0  (BUILD SUCCESS)

Note on -Djacoco.skip=true: this environment only has JDK 25 available, and the repo-pinned JaCoCo 0.8.7 cannot instrument Java 25 class files (Unsupported class file major version 69), which fails coverage instrumentation on every test regardless of changes. JaCoCo coverage was therefore skipped for local validation; compilation and all unit tests (including the 2 new carousel tests) pass.

No .changeset/ exists in this repo (Maven build, versioned via release tooling), so no changeset was added.

🤖 Generated with Claude Code

Co-Authored-By: Claude svc-devxp-claude@slack-corp.com

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 73.30%. Comparing base (a312395) to head (c052e00).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...om/slack/api/util/json/GsonLayoutBlockFactory.java	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1608      +/-   ##
============================================
- Coverage     73.32%   73.30%   -0.03%     
- Complexity     4520     4521       +1     
============================================
  Files           478      478              
  Lines         14300    14304       +4     
  Branches       1490     1490              
============================================
  Hits          10486    10486              
- Misses         2923     2926       +3     
- Partials        891      892       +1     

Flag	Coverage Δ
jdk-14	73.30% <75.00%> (-0.03%)	⬇️

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[zimeg]

Closing in favor of one-block-per-PR: card is #1609, carousel is now #1614. This bundled both, so superseding it.