ci(ci.yaml): add go-test job running go test -race ./...

[YimingIsCOLD]

🚀 Summary

CI currently verifies formatting, JS lint, and Go static analysis but does not execute Go tests, so a failing test only surfaces locally or after merge. This adds a go-test job that runs the suite with the race detector on every PR, catching concurrency bugs in the net/http handlers and middleware that static analysis cannot.

✏️ Changes

• Add go-test job to .github/workflows/ci.yaml, placed after go-lint.
• Job runs go test -race ./... on ubuntu-latest, using actions/setup-go@v6 with go-version-file: 'go.mod' (matches the go-lint job's preamble).
• Standalone parallel job with no needs:, matching the existing one-tool-per-job pattern.
• Relies on setup-go@v6's default module + build cache (keyed on go.sum); no explicit cache step.
• No coverage reporting, no OS/Go matrix, no custom timeout (deferred until a concrete need arises).

🧪 Test Plan

• go test -race ./... passes locally on all six test packages (handler, htmlutil, middleware, dotenv, random).
• The new Go Test check appears and passes on this PR.