Fix broken tests in test_registry (#707)

Summary:
Tests were broken after registry.get return type was changed from
`List[RoutableObjectWithProvider]` to
`Optional[RoutableObjectWithProvider]` in
efe791bab7 (diff-5de152bae521b7baef01048a4c0142484f8f1c978a04f3b55f4e4dabc52835beL29)

Test Plan:
Run tests
```
pytest llama_stack/distribution/store/tests/test_registry.py -v

collected 6 items

llama_stack/distribution/store/tests/test_registry.py::test_registry_initialization PASSED                                                                  [ 16%]
llama_stack/distribution/store/tests/test_registry.py::test_basic_registration PASSED                                                                       [ 33%]
llama_stack/distribution/store/tests/test_registry.py::test_cached_registry_initialization PASSED                                                           [ 50%]
llama_stack/distribution/store/tests/test_registry.py::test_cached_registry_updates PASSED                                                                  [ 66%]
llama_stack/distribution/store/tests/test_registry.py::test_duplicate_provider_registration PASSED                                                          [ 83%]
llama_stack/distribution/store/tests/test_registry.py::test_get_all_objects PASSED                                                                          [100%]
```

llama_stack/distribution/store/tests/test_registry.py

@@ -15,7 +15,8 @@ from llama_stack.distribution.store.registry import (
     CachedDiskDistributionRegistry,
     DiskDistributionRegistry,
 )
-from llama_stack.providers.utils.kvstore import kvstore_impl, SqliteKVStoreConfig
+from llama_stack.providers.utils.kvstore import kvstore_impl
+from llama_stack.providers.utils.kvstore.config import SqliteKVStoreConfig
 
 
 @pytest.fixture
@@ -26,14 +27,14 @@ def config():
     return config
 
 
-@pytest_asyncio.fixture
+@pytest_asyncio.fixture(scope="function")
 async def registry(config):
     registry = DiskDistributionRegistry(await kvstore_impl(config))
     await registry.initialize()
     return registry
 
 
-@pytest_asyncio.fixture
+@pytest_asyncio.fixture(scope="function")
 async def cached_registry(config):
     registry = CachedDiskDistributionRegistry(await kvstore_impl(config))
     await registry.initialize()
@@ -64,8 +65,8 @@ def sample_model():
 @pytest.mark.asyncio
 async def test_registry_initialization(registry):
     # Test empty registry
-    results = await registry.get("nonexistent", "nonexistent")
-    assert len(results) == 0
+    result = await registry.get("nonexistent", "nonexistent")
+    assert result is None
 
 
 @pytest.mark.asyncio
@@ -75,18 +76,16 @@ async def test_basic_registration(registry, sample_bank, sample_model):
     print(f"Registering {sample_model}")
     await registry.register(sample_model)
     print("Getting bank")
-    results = await registry.get("memory_bank", "test_bank")
-    assert len(results) == 1
-    result_bank = results[0]
+    result_bank = await registry.get("memory_bank", "test_bank")
+    assert result_bank is not None
     assert result_bank.identifier == sample_bank.identifier
     assert result_bank.embedding_model == sample_bank.embedding_model
     assert result_bank.chunk_size_in_tokens == sample_bank.chunk_size_in_tokens
     assert result_bank.overlap_size_in_tokens == sample_bank.overlap_size_in_tokens
     assert result_bank.provider_id == sample_bank.provider_id
 
-    results = await registry.get("model", "test_model")
-    assert len(results) == 1
-    result_model = results[0]
+    result_model = await registry.get("model", "test_model")
+    assert result_model is not None
     assert result_model.identifier == sample_model.identifier
     assert result_model.provider_id == sample_model.provider_id
 
@@ -103,9 +102,8 @@ async def test_cached_registry_initialization(config, sample_bank, sample_model)
     cached_registry = CachedDiskDistributionRegistry(await kvstore_impl(config))
     await cached_registry.initialize()
 
-    results = await cached_registry.get("memory_bank", "test_bank")
-    assert len(results) == 1
-    result_bank = results[0]
+    result_bank = await cached_registry.get("memory_bank", "test_bank")
+    assert result_bank is not None
     assert result_bank.identifier == sample_bank.identifier
     assert result_bank.embedding_model == sample_bank.embedding_model
     assert result_bank.chunk_size_in_tokens == sample_bank.chunk_size_in_tokens
@@ -129,18 +127,16 @@ async def test_cached_registry_updates(config):
     await cached_registry.register(new_bank)
 
     # Verify in cache
-    results = await cached_registry.get("memory_bank", "test_bank_2")
-    assert len(results) == 1
-    result_bank = results[0]
+    result_bank = await cached_registry.get("memory_bank", "test_bank_2")
+    assert result_bank is not None
     assert result_bank.identifier == new_bank.identifier
     assert result_bank.provider_id == new_bank.provider_id
 
     # Verify persisted to disk
     new_registry = DiskDistributionRegistry(await kvstore_impl(config))
     await new_registry.initialize()
-    results = await new_registry.get("memory_bank", "test_bank_2")
-    assert len(results) == 1
-    result_bank = results[0]
+    result_bank = await new_registry.get("memory_bank", "test_bank_2")
+    assert result_bank is not None
     assert result_bank.identifier == new_bank.identifier
     assert result_bank.provider_id == new_bank.provider_id
 
@@ -170,10 +166,10 @@ async def test_duplicate_provider_registration(config):
     )
     await cached_registry.register(duplicate_bank)
 
-    results = await cached_registry.get("memory_bank", "test_bank_2")
-    assert len(results) == 1  # Still only one result
+    result = await cached_registry.get("memory_bank", "test_bank_2")
+    assert result is not None
     assert (
-        results[0].embedding_model == original_bank.embedding_model
+        result.embedding_model == original_bank.embedding_model
     )  # Original values preserved