Fix JsonResponseBuilder treating application/vnd.elasticsearch+json as non-JSON

[Mpdreamz]

Summary

• JsonResponseBuilder and DynamicResponseBuilder both gated JSON parsing on contentType.StartsWith("application/json"). When Elasticsearch returns application/vnd.elasticsearch+json (its default when the client sends a matching Accept header), that check fails and the body is wrapped as a raw string under {"body": "..."} instead of being parsed as a JsonNode.
• This broke ServerReindex.StartReindexAsync in Elastic.Ingest.Elasticsearch: val.Get<string>("task") returned null → "Reindex response did not contain a 'task' field."

Changes

• ProductRegistration: new virtual IsJsonContentType(string?) — defaults to application/json prefix match.
• ElasticsearchProductRegistration: overrides to also accept application/vnd.elasticsearch+json (and ;compatible-with=N variants); +x-ndjson and binary subtypes remain unaffected.
• JsonResponseBuilder / DynamicResponseBuilder: replace hardcoded StartsWith("application/json") with boundConfiguration.ConnectionSettings.ProductRegistration.IsJsonContentType(contentType).
• JsonResponseTests: two new theory tests — one confirming vendor JSON types are parsed, one confirming non-JSON vendor types still fall through to raw-string mode.

Test plan

• BuilderElasticsearchVendorJsonContentType — application/vnd.elasticsearch+json (bare, ;compatible-with=8, with space) all parse the JSON body correctly
• BuilderElasticsearchVendorNonJsonContentType — +x-ndjson and +vnd.mapbox-vector-tile still wrap as raw string under body
• All existing tests pass (205/205 on net10.0, 202/203 on net481 — the one net481 failure is pre-existing and unrelated)

🤖 Generated with Claude Code

[flobernd]

LGTM!

Maybe we can also redirect this method

elastic-transport-net/src/Elastic.Transport/Products/Elasticsearch/ElasticsearchProductRegistration.cs

Line 403 in 5fb25bd

public override bool IsErrorContentType(string? contentType) =>

to the new IsJsonContentType.