fix(resilience): derive the chat_admission_busy Retry-After from observed lease occupancy (#12395)

The retryable chat_admission_busy 503 advertised a fixed Retry-After of 1s or 2s while the heavyweight lease it waits on is held for the entire SSE lifetime. Clients that honour the header — Codex CLI, agent fan-out — re-sent the same ~1 MiB /v1/responses body every second into a gate that could not have cleared, producing the queue_timeout retry storm that persisted even after OMNIROUTE_CHAT_MAX_HEAVY_IN_FLIGHT was raised.

ChatAdmissionController now tracks each live heavy lease's acquisition time and derives the hint from observed occupancy: the larger of the queue window the waiter already exhausted and the age of the youngest live lease, rounded up and capped at 60s. Both builders floor it at the historical 1s / 2s, so an idle gate answers exactly as before. Using the youngest rather than the oldest lease avoids a pessimistic hint when several slots are in flight.

Validated in a combined worktree with all 25 PRs of this batch boarded together: typecheck:core clean, 443/443 node-runner tests plus 14/14 vitest across every test file the batch touches, and check-changelog-integrity, check:cycles (418 files), check:provider-consistency (272 REGISTRY entries, 355 canonical providers), check:docs-counts, check:docs-sync (42 locales) and check-file-size all green.

Thanks @pacocartones.