feat(block-kit): add data visualization block example

[zimeg]

Summary

Adds a Block Kit example for the net-new data_visualization block to the block-kit showcase, mirroring the official docs example. Includes three meaningful chart variants:

• example01() — pie chart (segments)
• example02() — bar chart (series + axis_config)
• example03() — line chart (multiple series + axis_config)

slack_sdk does not yet ship a typed class for this block (confirmed absent in both 3.40.0 and the pinned 3.42.0), so the example defines a small DataVisualizationBlock(Block) subclass following the SDK's own convention (type, attributes property, keyword-only __init__). This keeps the example fully typed and mypy-clean while producing the correct to_dict() payload.

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

Changes

• block-kit/src/blocks/data_visualization.py — new example with pie/bar/line variants
• block-kit/tests/blocks/test_data_visualization.py — deep-equality tests against expected literals
• block-kit/README.md — new bullet (alphabetical) with docs + implementation links

Validation

Ran the same checks CI runs (.github/workflows/test.yml) for the block-kit showcase:

• ruff check — passed
• ruff format --diff --check — passed (already formatted)
• mypy — passed (no issues)
• pytest -v (new file) — 3 passed
• pytest (full block-kit suite) — 35 passed

Notes

• The line chart's second series uses distinct values; the docs example duplicated the first series' values verbatim, which appears to be a copy-paste artifact.
• One block per PR; opened as draft, not marking ready for review.

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

[zimeg]

Closing as duplicate of #101 (same data_visualization block example) — consolidating to one PR per block.