[AGiorgetti]

Summary

• Preserve application serializer and OnNextAsync Mongo exceptions in async observer reads by default; continue translating adapter Mongo failures.
• Add MongoPersistenceOptions.WrapAsyncApplicationExceptions = true to restore stable 12.1.0 wrapping.
• Add Breaking Changes and migration notes, including the custom protected-guard override caveat.

Closes #108.

Validation

• Test-first: 40 expected failures before the implementation.
• Cloud: 520 focused [P3] Review async exception translation for serializer and observer failures #108 tests passed on .NET 10 with MongoDB Driver 3.13.0, zero skipped, including suspended callback faults, stack trace, early stop and opt-out coverage.
• Earlier cancellation and regression run: 453 passed, zero skipped.
• Whitespace/diff check passed after transfer to an isolated local checkout.
• Driver 3.0.0 verification was blocked during restore by MSBuild named-mutex initialization. Other target frameworks and full integration suite remain unverified.

Compatibility / review notes

• Public/protected signatures and one guard invocation are preserved.
• Custom TryMongoAsync overrides that inspect callback exceptions can see an internal marker by default. Non-base overrides should opt out to preserve the previous callback contract.
• Per-item async helper overhead has not been benchmarked; no per-item delegate is introduced.

Co-authored-by: Alessandro Giorgetti alessandro.giorgetti@live.com
Co-authored-by: Codex (OpenAI) codex@openai.com

[AGiorgetti]

Refactored the per-item observer helpers at the owner's request:

• Successfully completed observer tasks are returned directly, without an additional async wrapper.
• Legacy opt-out forwards the original task, including pending/faulted/canceled tasks.
• A shared async slow path still marks deferred application Mongo exceptions. Synchronous conversion/callback exceptions remain guarded; outer virtual guard and API signatures are unchanged.
• Added task-identity and deterministic delayed true/false tests. Existing exception identity, delayed failures, cancellation and notification tests remain passing.

Verification: 14 new identity tests failed before the change; final focused regression suite has 587 tests passing with zero skipped in all eight net472/net8/net9/net10 × Driver 3.0.0/3.13.0 lanes. Full net10/Driver 3.13.0 suite against isolated MongoDB 8: 1,680 passed, zero skipped. Independent review found no correctness blocker. No benchmark was run, so no quantified throughput/allocation improvement is claimed. Existing protected-override caveat is unchanged.

Co-authored-by: Alessandro Giorgetti alessandro.giorgetti@live.com
Co-authored-by: Codex (OpenAI) codex@openai.com