_read_regular_file fdopen leaks

Trap: passing an os.open() descriptor directly to os.fdopen() looks like ownership transfer, but fdopen() can fail—directories raise IsADirectoryError, and injected failures such as ENOMEM leak the raw descriptor. Fix: classify and own the raw descriptor under try/finally until fdopen() succeeds; regressions must require controlled directory rejection and verify EBADF after directory and injected fdopen() failures.