Add SemanticSearcher for semantic search

[abetomo]

It basically just adds language_model_knn() to the Searcher's filter.
Overrode the keyword search only parts (e.g. match_columns) to do nothing.

[Copilot]

Pull request overview

This PR introduces a new FullTextSearch::SemanticSearcher intended to run semantic (embedding/KNN-based) search via PGroonga’s language_model_knn(), and updates the existing select plumbing so callers can target a specific PGroonga index.

Changes:

• Add SemanticSearcher (subclass of Searcher) that builds a semantic KNN filter and targets a dedicated semantic index.
• Extend the Searcher/backend select APIs to allow passing an explicit index_name.
• Add a unit test for semantic search (guarded behind SEMANTIC_SEARCH_TEST and PostgreSQL).

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
lib/full_text_search/semantic_searcher.rb	Adds semantic-search-specific searcher that uses language_model_knn and a semantic index.
test/unit/full_text_search/semantic_searcher_test.rb	Adds an opt-in test that provisions the semantic index and asserts semantic ranking.
lib/full_text_search/searcher.rb	Allows subclasses to override index_name and slice usage; passes index_name into Target.select.
lib/full_text_search/pgroonga.rb	Updates select/build_sql to support choosing a PGroonga index name.
lib/full_text_search/mroonga.rb	Updates select signature to accept (and ignore) index_name for API compatibility.

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[abetomo]

In the initial implementation, K is fixed at 50.

[kou]

How is this class used?

[abetomo]

This is how it's used.

--- a/lib/full_text_search/hooks/controller_search_index.rb
+++ b/lib/full_text_search/hooks/controller_search_index.rb
@@ -29,7 +29,12 @@ module FullTextSearch
           return
         end
 
-        searcher = Searcher.new(@search_request)
+        if @search_request.semantic? && SemanticIndex.exist?
+          searcher = SemanticSearcher.new(@search_request)
+        else
+          searcher = Searcher.new(@search_request)
+        end
+
         @result_set = searcher.search