Issue #3586543: Add AI reranking processor for Search API.

Description

A new Search API processor plugin (ai_reranker) that re-orders query results using the rerank operation type already in AI Core.

The rerank operation was added in #3488114 (closed) but nothing consumed it for Search API. #3557901 proposed putting reranking in ai_search, but reranking is a generic capability. It belongs in the ai core module where the operation type itself lives, so it works with any Search API backend (database, Solr, anything), not just the vector backend.

New files:

• src/Plugin/search_api/processor/AiReranker.php: ProcessorPluginBase plugin running at postprocess_query. Reads source fields from each result item, calls provider->rerank(), re-sorts items by the returned relevance scores, and applies an optional top-N slice. Logs a warning and returns the original result set if the rerank call fails.
• tests/src/Kernel/Plugin/Processor/AiRerankerTest.php: 7 Kernel tests covering plugin discovery, config round-trip, result reordering, top-N truncation, and three graceful-skip cases (no query keys, empty result set, missing provider config).

Modified:

• tests/modules/ai_test/src/Plugin/AiProvider/EchoProvider.php: adds ReRankInterface with deterministic descending mock scores so the Kernel tests have a provider to call.

No new runtime dependency on search_api. The plugin is only discovered when Search API is installed. drupal/search_api was already under require-dev in composer.json, so no change is needed there.

Closes #3586543

Testing instructions

Prerequisites:

• Enable search_api and search_api_db
• Install and configure a rerank-capable AI provider (e.g. Cohere via ai_provider_cohere) with a valid API key
• Create a Search API server (Database backend) and an index with a node datasource, title and body fields indexed

Configuration:

1. Open the index > Processors tab
2. Check AI Reranker to enable it
3. Select a rerank provider and model from the dropdown
4. Set Top N (e.g. 5) and check Title and Body as source fields
5. Save. No errors expected.

Reranking:

6. Run a search via a View or the Search API test page with a keyword
7. Results are re-ordered by semantic relevance, not keyword rank
8. With Top N set, the result count is capped to that number

Graceful skip:

9. Browse the view without a search term. All results display normally, nothing logged.

Checklist

• I have linked the related issue in the MR title or description
• I have performed a self-review of my own code
• I have added or updated tests, or explained in the description why this change is not covered by tests
• I have updated documentation for any new or changed functionality
• I have written testing instructions and verified them locally
• I have noted any required post-merge steps (config imports, cache rebuilds, manual changes)
• This MR contains no breaking API or hook changes, or they are explicitly documented in the description

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