feat: add the user_connections feature to the app manifest types

[WilliamBergamin]

Summary

This pull request adds the features.user_connections app manifest property (is_enabled, auth_hosts) to the manifest types, matching slackapi/slack-cli#688, so apps that opt into the standardized user connection flow can pass it to apps.manifest.create, update and validate without a type error.

Both fields are required, like Slack's manifest API, which rejects a user_connections object missing either one. Host format and item limits (1 to 25 hosts, no scheme) are validated by Slack.

Testing

• New tsd cases in the apps.manifest.create section of test/types/methods/apps.test-d.ts: missing is_enabled or auth_hosts is an error; enabled, and disabled with hosts, are assignable. (npm run test:types --workspace=packages/web-api; the only failures are the existing files.upload and rtm.start cases, also failing on main.)
• npm run lint and npm test --workspace=packages/web-api pass.

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.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.46%. Comparing base (2f5954b) to head (e3c223e).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2775      +/-   ##
==========================================
+ Coverage   89.38%   89.46%   +0.07%     
==========================================
  Files          65       65              
  Lines       10525    10525              
  Branches      478      478              
==========================================
+ Hits         9408     9416       +8     
+ Misses       1086     1078       -8     
  Partials       31       31              

Flag	Coverage Δ
cli-hooks	89.32% <ø> (ø)
cli-test	89.32% <ø> (ø)
logger	89.32% <ø> (ø)
oauth	89.32% <ø> (ø)
socket-mode	89.32% <ø> (ø)
web-api	89.32% <ø> (ø)
webhook	89.32% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.