Improve check for DBENGINE max datafile size allowed - #21390

Summary

• Handle race condition where multiple writers can obtain the same datafile and exceed the allowed max datafile size

Summary by cubic

Prevent datafiles from exceeding the configured max size by making the check atomic and accounting for the incoming extent size before writing. This eliminates the race where multiple writers could overfill a file.

• Bug Fixes
  • Check datafile capacity using pos + extent_size against the target size.
  • Lock the mutex at the start of datafile selection to make the check-and-create step atomic.
  • Pass the real I/O size to the selection logic to ensure accurate capacity checks.

^{Written for commit 02a6308. Summary will update automatically on new commits.}