test(reindex): add x-content round-trip tests for update and delete by

[wilmerdooley]

Adds AbstractXContentTestCase round-trip testing for UpdateByQueryRequest and DeleteByQueryRequest, alongside the already-tested ReindexRequest. A shared parseXContent helper in AbstractBulkByPaginatedSearchRequest applies request-specific top-level fields (conflicts, max_docs, and script for update) and then delegates the remaining body to SearchSourceBuilder. toXContent was extended on both requests to emit max_docs and conflicts so that serialization is symmetric with parsing, and the test instances now exercise the full state space.

Resolves #43456

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