fix(web-api): use json content type for chat stream methods

[zimeg]

Summary

This PR follows #1745 to use json instead of params as the content type for chat stream methods. Reference.

Fixes an issue where nested attributes - such as blocks - weren't parsed as expected. 🐛

Category

• slack_sdk.web.WebClient (sync/async) (Web API client)

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 (ai-apps@8519684). Learn more about missing BASE report.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@ Coverage Diff @@## ai-apps #1749 +/- ##
==========================================
Coverage ? 84.98% ==========================================
Files ? 113 Lines ? 12944 Branches ? 0 ==========================================
Hits ? 11000 Misses ? 1944 Partials ? 0 

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[mwbrooks]

✅ Looking good @zimeg! Thanks for following up on the previous PR feedback!

[zimeg]

@mwbrooks Appreciate it! Let's merge this with passing tests to unblock findings elsewhere 🧪 ✨