CSHARP-6089: Client Backpressure with baseBackoffMS#2065
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Update the exponential-backoff formula from 2^(attempt-1) to 2^attempt per the client-backpressure and transactions-convenient-api spec change (mongodb/specifications#1953), and update the dependent backoff tests. Also adds the baseBackoffMS overload integration test.
…and adapt tests to the new OperationContext API - Add Feature.ClientBackpressureBaseBackoffMs (wire version Server90) and gate the integration test on it instead of a server-version string, so it runs on 9.0 pre-release servers rather than skipping until GA. - Adopt the updated spec test 5 shape (absolute backoff bounds + assert the driver parsed baseBackoffMS) for the mocked unit test. - Fix GetOperationRetryBackoffDelay test bounds for the 2^attempt exponent. - Adapt tests to the CSHARP-6011 change (Session moved onto OperationContext; OperationContext(TimeSpan, CancellationToken) and NoTimeout removed).
The async retry path slept the full backpressure backoff via Task.Delay(backoff, operationContext.CancellationToken), which does not fire when the CSOT deadline elapses, so a large server-supplied baseBackoffMS could overshoot the deadline by seconds and surface a late TimeoutException. Add the same early-bail guard the sync path already has (throw the original overload error when the backoff would exceed the remaining timeout), matching the client-backpressure spec's deadline check. Add regression tests to both executor test suites and correct the AGENTS.md note that wrongly claimed the async path needed no guard.
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https://jira.mongodb.org/browse/CSHARP-6089
Implements spec PR mongodb/specifications#1953:
backpressurefield changes fromtrueto the literal string"2".retryAfterMSon an overload error overrides the default backpressure backoff base.Draft: the spec PR (#1953) is still open, and the server field is slated to be renamed
retryAfterMS→baseBackoffMS(SERVER-130142).This PR will need to be finalised after the spec PR has been merged. This includes updating the comments on tests.