Reject O_CREAT | O_EXCL before backend open side effects

The resolver currently invokes the backend open path before returning AlreadyExists for an existing O_CREAT|O_EXCL target. Backend-specific errors or side effects such as truncation can therefore occur before the exclusive-create result. Confirm the intended error precedence with the original fs author, then reject exclusive creation before backend open and add combined O_EXCL/O_TRUNC coverage. Deferred from #1318.

Trace the resolver's backend open path and inspect how existing O_CREAT|O_EXCL targets are handled before backend side effects occur. Confirm the intended error precedence with the original fs author, then verify that exclusive creation is rejected before backend open and add coverage for combined O_EXCL/O_TRUNC behavior.

Written by the indexing model from the issue text.