feat(model): add carousel Block Kit block

[zimeg]

Summary

Adds only the carousel Block Kit layout block (one block, one PR). A carousel is a horizontally scrollable collection of card blocks (1-10 cards).

Changes:

• New CarouselBlock model — TYPE = "carousel", elements: List<CardBlock> (1-10), blockId
• Blocks.carousel(...) builder factory
• GsonLayoutBlockFactory carousel case for (de)serialization
• BlockKitTest parse + build/round-trip coverage

Docs: https://docs.slack.dev/reference/block-kit/blocks/carousel-block

Dependency

CarouselBlock.elements is a List<CardBlock>, so this depends on the card block from #1609 (chore/block-kit-card). This PR is based on chore/block-kit-card, not main. Once #1609 merges to main, rebase this branch onto main and retarget the base to main.

This replaces the carousel half of the now-superseded #1608 (which bundled card + carousel). #1608 will be closed separately.

Validation

• ./mvnw -pl slack-api-model -Dtest=BlockKitTest test → Tests run: 47, Failures: 0, Errors: 0, Skipped: 0 — BUILD SUCCESS
• (Note: a JaCoCo coverage-instrumentation warning about "class file major version 69" appears under JDK 25; it is unrelated to this change and does not fail the build.)

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.32%. Comparing base (0712684) to head (fa3683f).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@                    Coverage Diff                     @@
##             chore/block-kit-card    #1614      +/-   ##
==========================================================
+ Coverage                   73.29%   73.32%   +0.03%     
- Complexity                   4521     4523       +2     
==========================================================
  Files                         478      478              
  Lines                       14302    14304       +2     
  Branches                     1490     1490              
==========================================================
+ Hits                        10482    10488       +6     
+ Misses                       2932     2924       -8     
- Partials                      888      892       +4     

Flag	Coverage Δ
jdk-14	73.32% <100.00%> (+0.03%)	⬆️

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