chore: order alphabetical error codes and constants in reference

[zimeg]

Changelog

N/A — no change to behavior but docs of #553 might keep new order 👁️‍🗨️ 📚

Summary

This pull request sorts the internal/slackerror/errors.go file alphabetically and ensures docgen produces deterministic output.

• Constants block sorted by ErrXxx identifier name
• ErrorCodeMap entries sorted by map key
• docgen now explicitly sorts errors before rendering the template

Preview

$ slack docgen docs/reference

Testing

• Confirm the removed diff of this PR has matching additions 🔬

Notes

The Go spec states (ref):

"The iteration order over maps is not specified and is not guaranteed to be the same from one iteration to the next."

Without explicit sorting, docgen output depends on undefined map iteration order, which can vary across Go versions and builds.

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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.70%. Comparing base (76e4649) to head (1a9fe50).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #569      +/-   ##
==========================================
+ Coverage   71.68%   71.70%   +0.01%     
==========================================
  Files         226      226              
  Lines       19144    19150       +6     
==========================================
+ Hits        13723    13731       +8     
+ Misses       4216     4214       -2     
  Partials     1205     1205              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.