Regenerate v5preview/opensearchapi for partial-failure errors

Mechanical output of cmd/osgen against the spec patched in the parent
commit. Two flavors of change:

  1. clients_gen.go now declares `errors errmask.ErrorMask` on Client
     and clientInit takes the mask as a second arg, matching the
     hand-written api.go callers from the parent commit.

  2. 19 operation dispatch files emit a per-wrapper post-do() block
     that returns a typed error when the corresponding errmask bit is
     unset and the wire data carries a partial failure. Wrappers
     covered today: BulkItems, SearchShards, WriteShards,
     MultiSearchItems. Wrappers in the catalog without hand-written
     emission yet (BroadcastShards, NodeFailures, BulkByScrollFailures,
     TaskFailures, MultiDocItems, Snapshot{Create,Get}ShardFailures,
     SimulateDocFailures, RankEvalFailures, IngestionShardFailures,
     PitNodeFailures) have their bits reserved by the spec annotation
     but produce no generated check until detection logic is added in
     a follow-up.

Operations whose typed response shape is missing the field path a
wrapper needs (CreateResp lacks _shards; msearch's union response
items lack a top-level Shards) are silently skipped by the dispatch
fragment's Applies guard. The annotation stays in the spec so the
emission will start automatically once the response types catch up.

Ref: #816

Signed-off-by: Sean Chittenden <sean.chittenden@crowdstrike.com>

Author: sean-