fix: skip codecov upload for dependabot prs

[eshaffer321]

Summary

• skip Codecov upload on Dependabot pull requests so the Test job reflects actual Go test results
• add a regression test for the CI workflow guard
• document the CI bug fix

Verification

• go test ./internal/ci -run TestCodecovUploadSkippedForDependabotPRs -v
• go test ./...

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.95%. Comparing base (d44dda6) to head (30bc81c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #57   +/-   ##
=======================================
  Coverage   59.95%   59.95%           
=======================================
  Files          47       47           
  Lines        5262     5262           
=======================================
  Hits         3155     3155           
  Misses       1920     1920           
  Partials      187      187           

Flag	Coverage Δ
unittests	59.95% <ø> (ø)

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

🚀 New features to boost your workflow:

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