feat(retrieval): rerank semantic search results

scripts/test-retrieval-server.py

@@ -0,0 +1,36 @@
+import importlib.util
+from pathlib import Path
+import unittest
+
+
+ROOT = Path(__file__).resolve().parent.parent
+SPEC = importlib.util.spec_from_file_location('retrieval_server', ROOT / 'shared/retrieval/server.py')
+MODULE = importlib.util.module_from_spec(SPEC)
+SPEC.loader.exec_module(MODULE)
+
+
+class RetrievalServerTests(unittest.TestCase):
+    def test_rerank_uses_cross_encoder_scores(self):
+        index = MODULE.Index.__new__(MODULE.Index)
+
+        class Reranker:
+            def predict(self, pairs):
+                self.pairs = pairs
+                return [0.1, 0.8, 0.5]
+
+        reranker = Reranker()
+        index.reranker = lambda: reranker
+        rows = [
+            {'path': 'a.md', 'score': 0.9, 'text': 'first'},
+            {'path': 'b.md', 'score': 0.8, 'text': 'second'},
+            {'path': 'c.md', 'score': 0.7, 'text': 'third'},
+        ]
+
+        result = index.rerank('query', rows, 2)
+
+        self.assertEqual([item['path'] for item in result], ['b.md', 'c.md'])
+        self.assertEqual(reranker.pairs, [('query', 'first'), ('query', 'second'), ('query', 'third')])
+
+
+if __name__ == '__main__':
+    unittest.main()