ci: use official Coveralls uploader

[raeperd]

Summary

• replace the community Go Coveralls uploader with Coveralls’ official v2 action
• explicitly upload coverage.txt as Go coverage data
• keep transient Coveralls service errors from blocking otherwise healthy CI

Why

The main workflow’s build, tests, lint, and dead-code checks passed, but Coveralls returned HTTP 500 (Build processing error) from /api/v1/jobs. Retrying the original job and testing Coveralls’ official reporter initially reproduced the same server error, confirming the failure was external to this repository. A later run uploaded coverage successfully.

Coverage generation remains required; only the external upload transport is non-blocking.

Validation

• make build VERSION=main
• git diff --exit-code go.mod go.sum
• make test
• make lint
• go run github.com/rhysd/actionlint/cmd/actionlint@latest .github/workflows/build.yaml
• PR checks: build and lint passed; Coveralls accepted the Go coverage report

[coveralls]

Coverage is 79.739% — codex/fix-coveralls-upload into main. No base build found for main.