[AGiorgetti]

Fixes #88.

Changes

• Acquire checkpoint cursors with FindAsync in both asynchronous helpers instead of FindSync.
• Preserve cancellation tokens, checkpoint selection, duplicate-ID resynchronization, and recycle-bin cleanup boundaries.
• Rely on the driver's FirstOrDefaultAsync cursor disposal, including failure and cancellation.
• Add 15 parameterized regression scenarios (45 assertions) covering default NextAsync, recycle cleanup, and duplicate-ID recovery with successful, empty, canceled-find, canceled-cursor, and failed-cursor outcomes.
• Update the changelog. No public API or persisted-format change.

Validation

• Static review and whitespace checks passed; remote head 4a64863 verified.
• Local builds and tests have not run. Earlier desktop restore was denied for unexpected instance-metadata access, and the cloud environment has no .NET SDK.
• Exact-head CI 41541602027 passed: Windows and Ubuntu solution builds; .NET 8/9/10 each passed 515 tests with zero failures or skips, including the 45 issue-88 regression tests.
• Two earlier CI attempts identified test-fixture problems (BSON assertion overload selection and the mocked DeleteManyAsync overload); both were corrected without changing the production fix. No pre-fix red test run is claimed.
• The existing test project resolves MongoDB.Driver 3.9, while the library declares 3.0. CI does not establish complete minimum-driver compatibility.
• Test-only FakeItEasy reference supports deterministic driver-seam regressions; no production dependency was changed.

Co-authored by Alessandro Giorgetti and Codex (OpenAI).