fix(audience-http): HTTP transport and flush hardening (SDK-234)

[ImmutableJeffrey]

Summary

• Locks HttpTransport backoff state behind _backoffLock. Readers (IsInBackoffWindow, NextAttemptAt, BackoffMs, RescheduleSendTimer) and writers (RecordFailure, ResetBackoff) all go through it.
• Stops HttpTransport.HttpClient from disposing a consumer-supplied handler (disposeHandler: false). Matches the existing _controlClient convention.
• Surfaces partial rejection: SendBatchAsync parses {accepted, rejected} from the 2xx body and fires OnError(ValidationRejected, ...) with the count when rejected > 0. Malformed body falls through as zero-rejected so a diagnostic parse error never blocks the success path.
• Gates FlushAsync on the existing _sendInFlight flag (the same gate the timer tick uses). Two parallel callers no longer double-POST the same batch; the second polls via Task.Yield until the first releases.
• Adds optional CancellationToken cancellationToken = default to FlushAsync (no behaviour change for existing callers) and catches ObjectDisposedException thrown when a concurrent Shutdown disposes the transport mid-flush.
• Adds 4 tests — concurrent-flush serialisation, partial-rejection warning on 200 with rejected > 0, zero-rejected silence, malformed-body fall-through.

Linear: SDK-234

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Note

Medium Risk
Touches the SDK’s event flushing/network send path, adding new concurrency and cancellation behavior plus backoff locking; regressions could affect delivery, retries, or shutdown behavior under load.

Overview
Hardens event flushing and HTTP transport behavior to avoid duplicate sends and improve observability.

ImmutableAudience.FlushAsync now accepts an optional CancellationToken, serializes concurrent flushes using the existing _sendInFlight gate (preventing double-POST of the same on-disk batch), and exits cleanly if a concurrent Shutdown disposes the transport.

HttpTransport now (1) locks all backoff state reads/writes, (2) avoids disposing a consumer-provided HttpMessageHandler, (3) surfaces partial 2xx acceptance by parsing rejected from the response body and firing OnError(ValidationRejected, ...), and (4) rethrows caller-initiated cancellation so flush loops don’t hot-spin. Tests were added to cover concurrent flush serialization, cancellation propagation, and the new 200-with-rejected response handling.

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