Detailed steps to reproduce the problem?

The previous implementation of Truncate missed the case where width falls exactly at the end of a multi-byte UTF-8 character.

F.E

Truncate("a©bc", 3) // © is encoded as 2 bytes: 0xC2 0xA9

str[:3] ends cleanly after ©, but the old logic checked the last byte (0xA9, pattern 10xxxxxx) and incorrectly treated it as a dangling continuation byte, stepping back past the complete © and returning "a" instead of the correct "a©".

The same bug affected the expected values in document_test.go and value_test.go, which were written against the incorrect behaviour.

See PR https://github.com/mongodb/mongo-go-driver/pull/2564

Definition of done: what must be done to consider the task complete?

• Fix Truncate in internal/bsoncoreutil to correctly handle the case where width falls exactly on a UTF-8 multi-byte character boundary.

The exact Go version used, with patch level:

$ go version

The exact version of the Go driver used:

$ go list -m go.mongodb.org/mongo-driver

Describe how MongoDB is set up. Local vs Hosted, version, topology, load balanced, etc.

N/A

The operating system and version (e.g. Windows 7, OSX 10.8, ...)

N/A