ci: add Node 26 to the test matrix

[zimeg]

Summary

This pull request adds Node 26 to the Node Slack SDK CI test matrix.

• Adds 26.x to the node-version matrix in .github/workflows/ci-build.yml (LATEST_SUPPORTED_NODE stays at 24.x).

Split out from #2634 so the Dependabot/config changes can land independently if the Node 26 test leg needs work.

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.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 79f3580

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.90%. Comparing base (70cec0c) to head (79f3580).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2635   +/-   ##
=======================================
  Coverage   88.90%   88.90%           
=======================================
  Files          63       63           
  Lines       10256    10256           
  Branches      452      452           
=======================================
  Hits         9118     9118           
  Misses       1117     1117           
  Partials       21       21           

Flag	Coverage Δ
cli-hooks	88.90% <ø> (ø)
cli-test	88.90% <ø> (ø)
logger	88.90% <ø> (ø)
oauth	88.90% <ø> (ø)
socket-mode	88.90% <ø> (ø)
web-api	88.90% <ø> (ø)
webhook	88.90% <ø> (ø)

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[zimeg]

📠 A comment for this addition. And the excellent reviewers!

[zimeg]

🎃 note: We don't require this check until LTS is reached around October IIRC!

Node.js 26 will enter long-term support (LTS) in October

🔗 https://nodejs.org/en/blog/release/v26.0.0/

[zimeg]

@mwbrooks For now let's merge this! I'm alright reverting if we find issues ongoing but hope we can keep packages stable toward these next updates 🐙 ✨