ci: gate PR merges on per-package coverage regression

[Surabhi-1605]

Summary

Adds a per-package coverage regression gate to consul-dataplane, ensuring no PR can lower the average test coverage of any package below its current baseline on main.

This complements the existing 90% patch coverage gate (merged in #1178) — patch coverage asks "are your new lines tested?", this gate asks "did the overall package health go down?"

How it works

1. On every merge to main, reusable-coverage-report.yml commits the per-package coverage averages to .github/coverage-baseline.txt (the authoritative baseline).
2. On every PR, the new coverage-regression-gate job downloads the per-package summary artifact, compares each package against the baseline, and fails if any package average dropped.
3. New packages (no prior baseline) are always allowed through.
4. On the first PR after this merges the gate skips gracefully — no baseline exists yet. The baseline is created on the next push to main.

Changes

• reusable-coverage-report.yml:

  • Upload pkg-summary artifact so the regression gate job can consume it
  • On merges to main, commit pkg-summary.txt as .github/coverage-baseline.txt
  • Fix coverage-gate if: expression (remove erroneous ${{ }} wrapper on job-level condition)

• consul-dataplane-checks.yaml:

  • Add coverage-regression-gate job that enforces per-package coverage does not regress

Notes

• Single module repo (github.com/hashicorp/consul-dataplane) — no multi-module complexity
• Mocks excluded (/mocks/, /mock_) consistent with existing coverage filtering
• Make coverage-regression-gate a required branch-protection check to enforce on merge

[github-actions]

Go Test Coverage: 66.7%

Patch coverage: N/A (no coverable Go lines changed)

See the workflow run for the full per-package breakdown and downloadable HTML report.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.48%. Comparing base (fefa27a) to head (363f080).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1247   +/-   ##
=======================================
  Coverage   68.48%   68.48%           
=======================================
  Files          19       19           
  Lines        2186     2186           
=======================================
  Hits         1497     1497           
  Misses        587      587           
  Partials      102      102           

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