feat: Allow :memory: for kvstore (#3696)

## Test Plan
added unit tests

tests/unit/utils/kvstore/test_sqlite_memory.py

@@ -0,0 +1,30 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+
+from llama_stack.providers.utils.kvstore.config import SqliteKVStoreConfig
+from llama_stack.providers.utils.kvstore.sqlite.sqlite import SqliteKVStoreImpl
+
+
+async def test_memory_kvstore_persistence_behavior():
+    """Test that :memory: database doesn't persist across instances."""
+    config = SqliteKVStoreConfig(db_path=":memory:")
+
+    # First instance
+    store1 = SqliteKVStoreImpl(config)
+    await store1.initialize()
+    await store1.set("persist_test", "should_not_persist")
+    await store1.shutdown()
+
+    # Second instance with same config
+    store2 = SqliteKVStoreImpl(config)
+    await store2.initialize()
+
+    # Data should not be present
+    result = await store2.get("persist_test")
+    assert result is None
+
+    await store2.shutdown()