Signed-off-by: Denys Fedoryshchenko <denys.f@collabora.com>

PATCH /node/{node_id} endpoint:
- Fetches the existing node from DB
- Merges only the fields provided in the request (exclude_unset=True)
- Validates the node subtype via parse_node_obj
- Validates state transitions (same logic as PUT)
- Resets processed_by_kcidb_bridge on update (same logic as PUT)
- Publishes CloudEvent (same as PUT)

Example:
curl -s -X PATCH -H 'Content-Type: application/json' \
     -H 'Authorization: Bearer ...' \
     -d '{"state":"done","result":"pass"}' \
    'https://api.kernelci.org/latest/node/69c24ea76d1f8ea2ada0f485'

Signed-off-by: Denys Fedoryshchenko <denys.f@collabora.com>