Closes F5/F6. The NC PUT handler now evaluates conditional
preconditions before body ingestion and returns 412 Precondition
Failed when they fail:
- If-None-Match: * on an existing target → 412 (create-if-absent)
- If-None-Match with matching ETag → 412 (weak compare)
- If-Match: * with no current representation → 412
- If-Match with no listed ETag strong-matching the current → 412
The lookup that drives the precondition reuses the same query the
handler already needed for the 201-vs-204 distinction, so this adds
no extra DB round-trip. Rejected requests skip body ingestion
entirely so a 412 doesn't waste megabytes of bandwidth + disk I/O.
Test F5/F6 flipped from 'pinned current 204' to assert 412, plus
mirror cases F5b/F6b/F6c/F6d covering the legitimate-success paths
so logical-operator regressions can't slip past silently.
Edouard Vanbelle
3fba933741