fix: remove icon_url from url source block element and update chat stream variable

[zimeg]

Summary

This PR removes the icon_url from the url source block element to match the expected API and also updates an internal chat_stream variable for maintenance ongoing.

Testing

Build these changes and stream a message with this app:

$ slack create slack-samples/bolt-python-assistant-template --branch feat-ai-apps-thinking-steps

Category

• slack_sdk.web.WebClient (sync/async) (Web API client)
• slack_sdk.models (UI component builders)
• /docs (Documents)
• tests/integration_tests (Automated tests for this library)

Notes

No update was made for an append without either "chunks" or "markdown_text" following discussion of #1809. With logging set to "debug" we might find an alright hint that the buffer is unchanged:

...
DEBUG:slack_sdk.web.base_client:ChatStream appended to buffer: {"buffer_length": 0, "buffer_size": 256, "channel": "D0A86GYHLG5", "recipient_team_id": "T02A074M3U3", "recipient_user_id": "U04051AF9NJ", "thread_ts": "1769060200.942659"}
DEBUG:slack_sdk.web.base_client:ChatStream appended to buffer: {"buffer_length": 0, "buffer_size": 256, "channel": "D0A86GYHLG5", "recipient_team_id": "T02A074M3U3", "recipient_user_id": "U04051AF9NJ", "thread_ts": "1769060200.942659"}
DEBUG:slack_sdk.web.base_client:ChatStream appended to buffer: {"buffer_length": 0, "buffer_size": 256, "channel": "D0A86GYHLG5", "recipient_team_id": "T02A074M3U3", "recipient_user_id": "U04051AF9NJ", "thread_ts": "1769060200.942659"}
...

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.
⚠️ Please upload report for BASE (feat-ai-apps-thinking-steps@610de5b). Learn more about missing BASE report.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@                      Coverage Diff                       @@
##             feat-ai-apps-thinking-steps    #1822   +/-   ##
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