ci: upload test results using the recommended codecov action

[zimeg]

Summary

This PR uses the recommended codecov/codecov-action to both upload test coverage and test results 🧪 ✨

From the pages of the codecov/test-results-action action:

⚠️ Deprecation Warning ⚠️

This Action causes issues for users uploading test analytics to Codecov

🔗 https://github.com/codecov/test-results-action?tab=readme-ov-file#%EF%B8%8F-deprecation-warning-%EF%B8%8F

Testing

Hopes to fix ongoing issues in CI:

⏳ https://github.com/slackapi/python-slack-sdk/actions/workflows/tests.yml?query=event%3Aschedule
🤖 https://github.com/slackapi/python-slack-sdk/actions/runs/19200481199/job/54887556261

Category

• Other

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.

[zimeg]

📬 Writings to wonderful reviewers as always-

[zimeg]

🗣️ note: Similar to the changes of slackapi/bolt-python#1396 we're setting this to true with belief that the updated action is more stable!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.36%. Comparing base (0c21814) to head (676a2cf).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@ Coverage Diff @@## main #1787 +/- ##
==========================================
+ Coverage 84.32% 84.36% +0.03% 
==========================================
Files 115 115 Lines 12399 12399 ==========================================
+ Hits 10456 10460 +4 + Misses 1943 1939 -4 

☔ View full report in Codecov by Sentry.
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[WilliamBergamin]

Nice 💯

[zimeg]

@WilliamBergamin Kind thanks! This is such an interesting action to investigate 🔍 ✨