ci: add a PR gate that actually checks the generated code (#548)

This repo has had no GitHub Actions at all -- only a .travis.yml, and Travis
stopped serving open-source repos long ago. So PRs here run zero build and zero
tests, which means `mergeable_state: clean` on a codegen PR currently means
"nobody is checking", not "it passed".

What this adds
--------------
`.github/workflows/ci.yml` with three checks and a single `ci-gate` aggregator,
so branch protection only ever needs one required check name regardless of how
the jobs are rearranged later.

The gate keeps only what decides whether the generated SDK is usable:

  ci-syntax   `go build ./...` + gofmtcheck -- covers services/ (the generated
              packages). Also catches duplicate imports and duplicate
              declarations, which Go treats as compile errors.
  lint        `go vet ./...` -- real bug patterns (printf arg mismatch, lock
              copying, unused assignments), not style.
  test        `make test-cov`.

Coverage upload is a continue-on-error step inside `test` and is deliberately
not part of `ci-gate`: an upload credential problem must never turn a PR red.

Also fixes a test that could not pass reliably
----------------------------------------------
`TestClientTimeout` pointed the client at https://httpbin.org/delay/2, a
third-party public service. When that service is unavailable the assertions
fail -- measured: it returned 503 and the test failed. There is no
testing.Short() guard either, so `-short` cannot skip it. `Test_errorHandler`
had a second reference to the same host.

Both now use a local httptest server. The handler also selects on
r.Context().Done() so it returns as soon as the client times out and
srv.Close() does not block.

Removes .travis.yml, which describes a CI that has not run in years.

Verified locally
----------------
  make ci-syntax   exit 0   (and exit 2 on an injected syntax error,
                             and exit 2 on injected gofmt drift)
  make lint        exit 0
  make test-cov    exit 0   (was failing on TestClientTimeout before this)

Not included: a scheduled compatibility matrix. The declared floor in go.mod is
go 1.13 and I have not verified the current tree builds on it, so shipping that
matrix now would just produce a red nightly. Worth doing as a follow-up.