fix: propagate async cancellation and await observer callbacks (#86)

Changelog.md

@@ -10,10 +10,14 @@
 
 - Fix initialization races: serialize initialization with a lock and retain success only after all setup completes. A waiting or later caller retries after failure; successful calls remain idempotent. Same-thread recursive initialization fails explicitly. Use `System.Threading.Lock` on .NET 9+ and an object lock on older targets. [#85](https://github.com/NEventStore/NEventStore.Persistence.MongoDB/issues/85)
 
+- Propagate cancellation from async operations, including snapshot writes. Observer reads signal request cancellation through `OnCompletedAsync` with the original token; `OperationCanceledException` while the request token is active is reported through `OnErrorAsync`. Observer reads use `ObserveMongoAsync` to complete successfully outside the guarded operation, retaining the protected virtual `TryMongoAsync` guard and its existing success behavior for derived engines. Terminal callbacks are awaited, and callback outcomes reach the returned task. [#86](https://github.com/NEventStore/NEventStore.Persistence.MongoDB/issues/86)
+
 ### Breaking Changes
 
 - New recycled records use the synthetic stored `StreamId`. Older adapter binaries expose that value and cannot find these records by the original stream ID; raw readers that assume stored `StreamId` is original see the same change. Upgrade recycle-bin readers before deleting streams with the new format. Existing recycled records need no backfill. [#87](https://github.com/NEventStore/NEventStore.Persistence.MongoDB/issues/87)
 
+- Non-observer async callers must now handle `OperationCanceledException` (including `TaskCanceledException`) from operations that previously swallowed cancellation or returned success/default values; for example, `AddSnapshotAsync` now propagates cancellation instead of returning `false`. Observer reads invoke `OnCompletedAsync` with the original token only when the request token is canceled; cancellation exceptions with an active request token go to `OnErrorAsync`. Cancellation or failure from a terminal callback propagates after it is awaited, without a second terminal notification. [#86](https://github.com/NEventStore/NEventStore.Persistence.MongoDB/issues/86)
+
 ### Migration notes
 
 - `Initialize()` now creates `GetFrom_RecycleBin_Index` on existing databases. No data backfill or old-index replacement is required, but deployments must budget index build time and disk/I/O costs and retain index-creation permission. Operators can pre-create the documented definition to move build work out of startup. See [recycle-bin rollout and indexing](docs/Recycle-Bin.md#recycle-lookup-index-and-deployment). [#87](https://github.com/NEventStore/NEventStore.Persistence.MongoDB/issues/87)