fix(models): type CardBlock icon/hero_image as image element (not str)

[zimeg]

Summary

The card block reference specifies that icon and hero_image are image element objects ({"type": "image", "image_url": ..., "alt_text": ...}), not plain URL strings. But CardBlock typed both as Optional[str], so passing an ImageElement — the correct, accessible form (it carries alt_text) — raised a mypy [arg-type] error and forced callers to use # type: ignore.

This widens both parameters to Optional[Union[str, dict, ImageElement]], matching how title/subtitle/body already accept Union[str, dict, TextObject].

Details

• slack_sdk/models/blocks/blocks.py — CardBlock.__init__: hero_image/icon typed Optional[Union[str, dict, ImageElement]]; docstrings updated to the reference wording (incl. the max-length notes).
• Runtime behavior is unchanged — serialization already handled all three forms: str → bare URL (back-compat, still covered by test_document), dict/ImageElement → the object shape. This is a type-widening only.
• Adds test_image_element_icon_and_hero_image asserting the ImageElement form round-trips to the documented object.

Testing

• pytest tests/slack_sdk/models/test_blocks.py — 85 passed (incl. the new test; no regressions).
• mypy slack_sdk/models/blocks/blocks.py — clean.

Motivation

Downstream: the Bolt bolt-python-examples card/carousel examples build icon/hero_image with ImageElement (docs-correct) and currently need # type: ignore[arg-type] because of this mis-typing. This fix lets those examples drop the suppression.

🤖 Generated with Claude Code

[zimeg]

Note on keeping str in the type

The card block reference documents icon/hero_image as image-element objects only — it never shows a bare URL string. So one could argue for typing these as Union[dict, ImageElement] (object-only).

This PR deliberately keeps str (Union[str, dict, ImageElement]) because:

• The string form is already shipped and tested (CardBlockTests.test_document passes icon/hero_image as URL strings and round-trips them). Dropping str would be a breaking change (semver:major) and would fail that test.
• This change is purely additive — it widens the accepted/typed input to include the docs-correct object form, without removing the existing behavior. That keeps it semver:minor.

If maintainers prefer to make these object-only, that'd be a separate major-version change (update test_document, drop str). Happy to follow up if that's the preference.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.17%. Comparing base (c2eacc2) to head (bb2a6a1).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1937   +/-   ##
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  Coverage   84.17%   84.17%           
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  Files         118      118           
  Lines       13425    13425           
=======================================
  Hits        11301    11301           
  Misses       2124     2124           

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[zimeg]

Leaving a callout on the breaking change that I'm marking as a patch update 📣

[zimeg]

⚠️ note: The str option is removed which typechecking might complain about but this matches what's valid with the API itself:

https://app.slack.com/block-kit-builder#%7B%22blocks%22:%5B%7B%22type%22:%22card%22,%22icon%22:%22https://picsum.photos/36/36%22,%22title%22:%7B%22type%22:%22mrkdwn%22,%22text%22:%22Lumon%20Industries%22,%22verbatim%22:false%7D,%22subtitle%22:%7B%22type%22:%22mrkdwn%22,%22text%22:%22Committed%20to%20work-life%20balance%22,%22verbatim%22:false%7D,%22hero_image%22:%22https://picsum.photos/400/300%22,%22body%22:%7B%22type%22:%22mrkdwn%22,%22text%22:%22Please%20enjoy%20each%20card%20equally.%22,%22verbatim%22:false%7D,%22actions%22:%5B%7B%22type%22:%22button%22,%22text%22:%7B%22type%22:%22plain_text%22,%22text%22:%22Action%20Button%22,%22emoji%22:false%7D,%22action_id%22:%22button_action%22%7D%5D%7D%5D%7D