Closes F11.
The NC MKCOL handler previously had `mkdir -p` semantics:
sending MKCOL on /a/b/c/ where neither a nor b exists silently
created both intermediates and returned 201. Sabre/DAV and the
actual NC server both 409 on that — our auto-create deviated.
NC desktop walks ancestors one MKCOL at a time during sync so
nothing real depended on the old behaviour.
Drop the segment-walking creation loop. New flow:
target exists → 405
parent path missing → 409
parent ok, target new → 201
The race-recovery branch for the loop's per-segment create is
also gone — single parent lookup, single create, no window.
Test F11 flipped from pinned-201 to strict 409 and asserts the
intermediate parent was not silently created. F11b and F11c added
as regression guards for the success path and the 'target already
exists' case.
Edouard Vanbelle
af3ce24062