Keep scroll contexts on transient search queue rejection

[reugn]

When a scroll continuation is rejected by a full search thread pool, SearchService.processFailure freed the scroll reader context. The rejection happens at submission before scroll state advances, so the client still has a usable scroll_id but retries fail because the context is gone.

The fix keeps the context on transient EsRejectedExecutionException (not executor shutdown) for InternalScrollSearchRequest query and fetch continuations only. If the initial scroll search is rejected before the client gets a scroll_id (including the DFS query phase), the context is still freed so it cannot leak until keep-alive expires. The legacy query-and-fetch scroll fetch path also retains on rejection; the new IT covers the usual scroll query path only.

Fixes #90912

[elasticsearchmachine]

Pinging @elastic/es-search-foundations (Team:Search Foundations)

[elasticsearchmachine]

Hi @reugn, I've created a changelog YAML for you.

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[github-actions]

ℹ️ Important: Docs version tagging

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