feat(clients/gitlab): add HealthcheckClient for /check endpoint

Adds `HealthcheckClient` to `internal/clients/gitlab` — a thin wrapper around `GET /api/v4/internal/check` using the new labkit HTTP client. This is the new-client implementation that the FF-gated dispatch (MR3) will call when `use_new_healthcheck_client` is enabled.

What this MR does

• Defines `HealthcheckResponse` and `HealthcheckClient` in `internal/clients/gitlab/healthcheck.go`
• `Check(ctx)` calls `client.Get` then delegates JSON parsing and error handling to the shared `ParseJSON` helper (which returns `*client.APIError` for non-2xx responses and `errors.New("parsing failed")` for invalid JSON)
• Extracts the endpoint path into a `healthcheckPath` package-level constant
• Tests live in `package gitlab_test`, use the shared `newTestClient` helper, and assert against concrete error types (`require.ErrorAs` / `require.EqualError`) rather than just `require.Error`

Test plan

``` go test ./internal/clients/gitlab/... ```

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Stack (merge in order)

#	MR	Status
1	!1465 (merged) — retry policy	merge into `main` first
2	!1466 (merged) — new HealthcheckClient ← you are here	retarget to `main` after MR1 merges
3	!1467 (merged) — FF-gated dispatch	retarget to `main` after MR2 merges
4	!1468 (merged) — acceptance test harness	retarget to `main` after MR3 merges