feat(python): list registered embedding functions

Closes #1605.

Adds `EmbeddingFunctionRegistry.list_functions()` to expose registered lookup names in registration order without accessing `_functions`. The method returns an independent snapshot, and every returned name can be passed to `get()`.

Tests cover empty registries, aliases and class names, registration order, snapshot isolation, and reset.

python/python/lancedb/embeddings/registry.py

@@ -101,6 +101,25 @@ class EmbeddingFunctionRegistry:
         """
         return self._functions[name]
 
+    def list_functions(self) -> list[str]:
+        """List the names of registered embedding functions.
+
+        Returns
+        -------
+        list[str]
+            Registered aliases, or class names for functions registered without
+            an alias, in registration order. Each name can be passed to
+            :meth:`get`. The returned list is a copy; changing it does not
+            change the registry.
+
+        Examples
+        --------
+        >>> registry = get_registry()
+        >>> "openai" in registry.list_functions()
+        True
+        """
+        return list(self._functions)
+
     def parse_functions(
         self, metadata: Optional[Dict[bytes, bytes]]
     ) -> Dict[str, "EmbeddingFunctionConfig"]:

python/python/tests/test_embedding_registry_listing.py

@@ -0,0 +1,55 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright The LanceDB Authors
+
+from lancedb.embeddings import EmbeddingFunctionRegistry, TextEmbeddingFunction
+
+
+class ExampleEmbedding(TextEmbeddingFunction):
+    def ndims(self):
+        return 2
+
+    def generate_embeddings(self, texts):
+        return [[1.0, 0.0] for _ in texts]
+
+
+class OtherEmbedding(ExampleEmbedding):
+    pass
+
+
+def test_list_functions_empty_registry():
+    registry = EmbeddingFunctionRegistry()
+    assert registry.list_functions() == []
+
+
+def test_list_functions_names_and_registration_order():
+    registry = EmbeddingFunctionRegistry()
+    registry.register("custom-embedding")(ExampleEmbedding)
+    registry.register()(OtherEmbedding)
+
+    assert registry.list_functions() == ["custom-embedding", "OtherEmbedding"]
+    assert [registry.get(name) for name in registry.list_functions()] == [
+        ExampleEmbedding,
+        OtherEmbedding,
+    ]
+
+
+def test_list_functions_returns_independent_snapshot():
+    registry = EmbeddingFunctionRegistry()
+    registry.register("first")(ExampleEmbedding)
+    names = registry.list_functions()
+    names.append("not-registered")
+    assert registry.list_functions() == ["first"]
+
+    registry.register("second")(OtherEmbedding)
+    assert names == ["first", "not-registered"]
+    assert registry.list_functions() == ["first", "second"]
+
+
+def test_list_functions_after_reset():
+    registry = EmbeddingFunctionRegistry()
+    registry.register("custom-embedding")(ExampleEmbedding)
+    names = registry.list_functions()
+    registry.reset()
+
+    assert registry.list_functions() == []
+    assert names == ["custom-embedding"]