test(opensearch3): expand HttpClient action coverage and fix 3 latent bugs

[marevol]

Overview

OpenSearch3ClientTest was already the most complete of the five backend integration suites, so this PR closes the remaining gap against the registered-action API surface: several Http*Actions wired in HttpClient had no coverage anywhere. Adding end-to-end tests for them (against a real OpenSearch 3.7 Testcontainer) surfaced three latent, previously-untested production bugs, which are fixed here.

New / enhanced tests (OpenSearch3ClientTest)

• Task management / diagnostics smoke tests: test_list_tasks, test_get_task, test_cancel_tasks, test_nodes_usage, test_remote_info, test_upgrade_status.
• Snapshot & repository lifecycle: test_crud_snapshot_repository (put → verify → get → delete) and test_snapshot_lifecycle (create → status → get → restore → delete, asserting the restored doc is searchable). startServer() now sets path.repo so an fs repository can be registered; teardown uses two independent delete blocks so one cleanup failure cannot leak the other resource.
• Engine detection: test_engine_info asserts the live client detects OPENSEARCH3.
• Depth improvements: test_pit_lifecycle now runs a PIT-backed search and asserts it reads the captured documents; test_stats now asserts the serialized NodesStatsResponse XContent is non-empty; test_upgrade_status asserts real (non-zero) byte totals.

Bug fixes surfaced by the new coverage

1. HttpPutRepositoryAction sent no request body.
PUT /_snapshot/{name} was issued without a body, so the server rejected it with 400 parse_exception: request body is required — the action never worked. It now serializes the repository type/settings via request.toXContent(...), mirroring HttpCreateSnapshotAction. Adds HttpPutRepositoryActionTest.

2. HttpUpgradeStatusAction always returned empty data.
fromXContent looked for a top-level _shards object that the _upgrade response never emits and always serialized zero ShardUpgradeStatus entries, so getIndices() was always empty and all byte/shard counts were 0. It also wrote the BroadcastResponse header with writeInt where the wire format reads writeVInt (only "worked" because every value was zero). It now parses the per-index aggregate byte counts (size_in_bytes / size_to_upgrade_in_bytes / size_to_upgrade_ancient_in_bytes) and reconstructs the response so getTotalBytes() and the per-index sizes are correct. Because the _upgrade API exposes no per-shard routing (detailed=true is rejected on 3.7), the reconstruction synthesizes one shard per index; the shard routing is therefore artificial and only the byte totals are meaningful — this is documented in the code. Adds HttpUpgradeStatusActionTest.

3. HttpSearchAction hung on point-in-time searches.
It emitted ccs_minimize_roundtrips and the index options (ignore_unavailable / allow_no_indices / expand_wildcards) unconditionally. OpenSearch rejects both together with a point-in-time search (400 "... cannot be used with point in time"), and over this HTTP client a 400 on a request that carries a body manifests as an indefinite hang. It now omits both when the request carries a PIT (a PIT already binds its own indices). Non-PIT searches are byte-identical, so all other engines are unaffected. Extends HttpSearchActionTest.

Testing

• OpenSearch3ClientTest: 88/88 green (Docker, OpenSearch 3.7.0).
• Offline unit suite (all engines' request-building + node/engine tests): 340/340 green.
• mvn formatter:format license:format applied.

Notes / out of scope

• The sibling HttpUpgradeAction (the POST /_upgrade action, no integration coverage here) has the same writeInt-vs-writeVInt header issue; left untouched to keep this change scoped.
• The PIT-search fix guards the two parameter groups OpenSearch actually rejects with a PIT; other search parameters are unchanged.