feat(block-kit): add card block example

[zimeg]

Summary

Adds a card block example to the Bolt for Python Block Kit examples, mirroring the official docs example (and the matching bolt-js-examples card example).

• block-kit/src/blocks/card.py
  • example01() — a full card with an icon, title, subtitle, hero image, body, and an action button.
  • example02() — a minimal card with only a title and an action button (at least one of hero_image, title, actions, or body is required).
• block-kit/tests/blocks/test_card.py — asserts each produced block deep-equals the expected literal.
• block-kit/README.md — adds a Card bullet (alphabetical) with docs + implementation links.

Docs

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

Notes

CardBlock.icon / CardBlock.hero_image are typed as Optional[str] in slack_sdk 3.42.0, but the documented example (and runtime serialization) uses image element objects. The example passes ImageElement to match the docs structure, with a narrow # type: ignore[arg-type] on those two arguments to satisfy mypy (the SDK type hint is narrower than the accepted input).

Validation

All four CI gates run locally against slack_sdk==3.42.0 (pinned in requirements.txt):

• ruff check — passed (All checks passed!)
• ruff format --diff --check — passed (34 files already formatted)
• mypy ./**/*.py — passed (Success: no issues found in 34 source files)
• pytest — passed (34 passed, including the 2 new test_card cases)

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

[zimeg]

Closing as duplicate of #102 (same card block example) — consolidating to one PR per block.