Add diversity-aware selection after Document-RAG reranking (#1014)

* Add Document-RAG diversity selection helper

* Add optional MMR diversity selection after reranking

* Fix Document-RAG diversity test method signatures

tests/unit/test_retrieval/test_document_rag_rerank.py

@@ -476,3 +476,75 @@ class TestRerankActive:
         await rag.query(query="What is the return policy?")
 
         assert reranker.calls == []
+
+# ---------------------------------------------------------------------------
+# 3. Diversity selection: optional MMR after cross-encoder scoring
+# ---------------------------------------------------------------------------
+
+    @pytest.mark.asyncio
+    async def test_diversity_mode_scores_full_candidate_pool_before_selecting(self):
+        """
+        With diversity selection enabled, the cross-encoder should score the full
+        fetched candidate pool before MMR narrows it down to doc_limit.
+        """
+        clients = build_mock_clients()
+        reranker = StubReranker([
+            RerankerResult(document_id="0", query_id="0", score=1.00),
+            RerankerResult(document_id="1", query_id="0", score=0.95),
+            RerankerResult(document_id="2", query_id="0", score=0.90),
+        ])
+        rag = DocumentRag(
+            *clients,
+            reranker_client=reranker,
+            rerank_diversity_mode="mmr",
+        )
+
+        await rag.query(query="What is the return policy?", doc_limit=2)
+
+        assert reranker.calls[0]["limit"] == len(ORDERED_CONTENT)
+
+        call = rag.prompt_client.document_prompt.call_args
+        passed_docs = call.kwargs["documents"]
+        assert len(passed_docs) == 2
+
+
+    @pytest.mark.asyncio
+    async def test_diversity_mode_selects_less_redundant_context_set(self):
+        """
+        MMR should use cross-encoder scores as relevance while penalizing redundant
+        chunks, so a slightly lower-scored but less redundant chunk can be selected.
+        """
+        clients = build_mock_clients()
+        prompt_client, embeddings_client, doc_embeddings_client, fetch_chunk = clients
+
+        duplicate_a = "apple banana fruit return policy"
+        duplicate_b = "apple banana fruit return policy duplicate"
+        diverse_c = "engine motor vehicle warranty"
+
+        async def mock_fetch(chunk_id):
+            return {
+                CHUNK_A: duplicate_a,
+                CHUNK_B: duplicate_b,
+                CHUNK_C: diverse_c,
+            }[chunk_id]
+
+        fetch_chunk.side_effect = mock_fetch
+
+        reranker = StubReranker([
+            RerankerResult(document_id="0", query_id="0", score=1.00),
+            RerankerResult(document_id="1", query_id="0", score=0.95),
+            RerankerResult(document_id="2", query_id="0", score=0.90),
+        ])
+        rag = DocumentRag(
+            *clients,
+            reranker_client=reranker,
+            rerank_diversity_mode="mmr",
+            rerank_diversity_lambda=0.2,
+        )
+
+        await rag.query(query="What is the return policy?", doc_limit=2)
+
+        call = rag.prompt_client.document_prompt.call_args
+        passed_docs = call.kwargs["documents"]
+
+        assert passed_docs == [duplicate_a, diverse_c]