feat(resilience): add hierarchical concurrency admission

docs/changelog/fragments/7778.md

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+feat(resilience): atomically enforce cumulative global, provider, and account concurrency limits

docs/plans/7778-hierarchical-admission.md

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+# #7778 hierarchical admission cleanup plan
+
+1. Lock the existing single-key semaphore contract and the new atomic multi-key
+   contract with focused tests: no partial reservations, FIFO queueing, abort,
+   timeout, queue-full, idempotent release, stats, and cleanup.
+2. Generalize the existing account semaphore in place. Keep `acquire()` as a
+   compatibility wrapper around `acquireMany()`; do not add a second scheduler
+   or a dependency.
+3. Replace the account-only acquisition in `chatCore` with one cumulative
+   global/provider/account acquisition immediately before `withRateLimit`.
+   Reacquire the whole set whenever account rotation changes the connection,
+   and retain the release through streaming completion.
+4. Extend the existing resilience settings pipeline (types, defaults,
+   normalization, schema, API response, UI, and translations) with the global
+   and provider caps. Relabel the old Bottleneck concurrency control as
+   connection/quota-scope concurrency so its real scope is explicit.
+5. Run focused tests, lint, typecheck, static checks, and the full test suite;
+   document the behavioral change in the changelog.
+
+Behavior intentionally preserved: zero/null concurrency bypasses a gate,
+account-only callers keep using `acquire()`, blocked-account controls retain
+their key format and API, and provider rate-limit queue behavior remains
+unchanged.