feat(v2/httpserver): include build info in health responses

What this MR does

Adds Version, Commit, and BuildDate fields to httpserver.Config. When set, these are included in the /-/liveness and /-/readiness JSON response bodies so operators can confirm which build is running without inspecting the binary directly.

Fields are omitted from the response when empty, so existing callers require no changes.

Config

httpserver.NewWithConfig(&httpserver.Config{
    Version:   app.Version,
    Commit:    app.Commit,
    BuildDate: app.BuildDate,
})

Response shape

{
  "status": "ok",
  "version": "v1.2.3",
  "commit": "abc1234",
  "build-date": "2026-05-07T00:00:00Z"
}

How to test

cd v2 && go test ./httpserver/... -v -run TestLiveness_IncludesBuildInfo -run TestReadiness_IncludesBuildInfo