mongodb: fix data race on ChangeStream between TryNext and ResumeToken

[mmatczuk]

The ack callback called stream.ResumeToken() from a different goroutine
than the one running stream.TryNext(), racing on the ChangeStream's
internal resume token field. Use the value already resolved by the
checkpoint system instead, which is both race-free and semantically
correct.

Fixes CON-418

[claude]

Commits
LGTM

Review
One-line fix replaces stream.ResumeToken() in the ack callback (running on a different goroutine) with *resumeToken from the checkpoint system's resolve(). The resume token was already captured safely in the main goroutine at track time, and the pointer is properly nil-guarded on line 915. Clean race fix.

LGTM