feat(models): add markdown block

[zimeg]

Summary

This PR adds the markdown block to fix#1688.

📚 Reference: https://docs.slack.dev/reference/block-kit/blocks/markdown-block/

Testing

The following commands can confirm typing as expected in a sample app:

$ slack create asdf -t slack-samples/bolt-python-getting-started-app
$ cd asdf
$ source .venv/bin/activate # Setup virtual environment
$ vim app.py # Update with contents below
$ slack run # Invite bot to channel and say "hello"

fromslack_sdk.models.blocksimportMarkdownBlock
...
@app.message("hello")defmessage_hello(say):
say(
blocks=[
MarkdownBlock(text="**lets's go!**"),
],
text="let's go!",
)

Category

• slack_sdk.models (UI component builders)

Notes

I'm finding the unit tests hang for RTM tests on my machine... Will confirm this passes in CI! 🤖

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.
• I've run python3 -m venv .venv && source .venv/bin/activate && ./scripts/run_validation.sh after making the changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.93%. Comparing base (ed1893d) to head (2d825d9).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@ Coverage Diff @@## main #1748 +/- ##
==========================================
+ Coverage 84.90% 84.93% +0.02% 
==========================================
Files 113 113 Lines 12886 12905 +19 ==========================================
+ Hits 10941 10961 +20 + Misses 1945 1944 -1 

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[mwbrooks]

✅ Implementation looks good to me!

🧪 Manual tests worked on my end. Thanks a bunch for the test setup steps!

[mwbrooks]

praise: 🔤 respect the alphabetized!

[zimeg]

@mwbrooks Thanks so much for the kind review and testing! I find the validation to match what's supported on the backend at this time so let's merge this 🚢 💨