fix(clients/gitlab): match legacy client retry policy

Adds retryConfig() to the new gitlab client so outbound requests use the same retry policy as the legacy hashicorp/go-retryablehttp client: 3 total attempts, retries on transient 5xx (all except 501), 429 Too Many Requests, and network errors — with backoff capped at 15s.

Without this, labkit's narrower default (only 429+503) caused silent behavioral divergence between the two client paths.

Test plan

• go test ./internal/clients/gitlab/...

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Stack (merge in order)

#	MR	Status
1	!1465 (merged) — retry policy ← you are here	merge into main first
2	!1466 (merged) — new HealthcheckClient	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