[AUR-420] Provide document store base interface and an in-memory version (#21)

Document store handles storing and indexing Documents. It supports the following interfaces:

- add: add 1 or more documents into document store
- get: get a list of documents
- get_all: get all documents in a document store
- delete: delete 1 or more document
- save: persist a document store into disk
- load: load a document store from disk

tests/test_docstores.py

@@ -0,0 +1,58 @@
+import pytest
+
+from kotaemon.docstores import InMemoryDocumentStore
+from kotaemon.documents.base import Document
+
+
+def test_simple_document_store_base_interfaces(tmp_path):
+    """Test all interfaces of a a document store"""
+
+    store = InMemoryDocumentStore()
+    docs = [
+        Document(text=f"Sample text {idx}", meta={"meta_key": f"meta_value_{idx}"})
+        for idx in range(10)
+    ]
+
+    # Test add and get all
+    assert len(store.get_all()) == 0, "Document store should be empty"
+    store.add(docs)
+    assert len(store.get_all()) == 10, "Document store should have 10 documents"
+
+    # Test add with provided ids
+    store.add(docs=docs, ids=[f"doc_{idx}" for idx in range(10)])
+    assert len(store.get_all()) == 20, "Document store should have 20 documents"
+
+    # Test add without exist_ok
+    with pytest.raises(ValueError):
+        store.add(docs=docs, ids=[f"doc_{idx}" for idx in range(10)])
+
+    # Update ok with add exist_ok
+    store.add(docs=docs, ids=[f"doc_{idx}" for idx in range(10)], exist_ok=True)
+    assert len(store.get_all()) == 20, "Document store should have 20 documents"
+
+    # Test get with str id
+    matched = store.get(docs[0].doc_id)
+    assert len(matched) == 1, "Should return 1 document"
+    assert matched[0].text == docs[0].text, "Should return the correct document"
+
+    # Test get with list of ids
+    matched = store.get([docs[0].doc_id, docs[1].doc_id])
+    assert len(matched) == 2, "Should return 2 documents"
+    assert [doc.text for doc in matched] == [doc.text for doc in docs[:2]]
+
+    # Test delete with str id
+    store.delete(docs[0].doc_id)
+    assert len(store.get_all()) == 19, "Document store should have 19 documents"
+
+    # Test delete with list of ids
+    store.delete([docs[1].doc_id, docs[2].doc_id])
+    assert len(store.get_all()) == 17, "Document store should have 17 documents"
+
+    # Test save
+    store.save(tmp_path / "store.json")
+    assert (tmp_path / "store.json").exists(), "File should exist"
+
+    # Test load
+    store2 = InMemoryDocumentStore()
+    store2.load(tmp_path / "store.json")
+    assert len(store2.get_all()) == 17, "Laded document store should have 17 documents"