Adding memory provider test fakes

Summary:
Part of
* https://github.com/meta-llama/llama-stack/issues/436

This change adds a minimalistic support to creating memory provider test fake. For more details about the approach, follow the issue link from above.

Test Plan:
Run tests using the "test_fake" mark:
```
pytest llama_stack/providers/tests/memory/test_memory.py -m "test_fake"
/opt/homebrew/Caskroom/miniconda/base/envs/llama-stack/lib/python3.11/site-packages/pytest_asyncio/plugin.py:208: PytestDeprecationWarning: The configuration option "asyncio_default_fixture_loop_scope" is unset.
The event loop scope for asynchronous fixtures will default to the fixture caching scope. Future versions of pytest-asyncio will default the loop scope for asynchronous fixtures to function scope. Set the default fixture loop scope explicitly in order to avoid unexpected behavior in the future. Valid fixture loop scopes are: "function", "class", "module", "package", "session"

  warnings.warn(PytestDeprecationWarning(_DEFAULT_FIXTURE_LOOP_SCOPE_UNSET))
====================================================================================================== test session starts ======================================================================================================
platform darwin -- Python 3.11.10, pytest-8.3.3, pluggy-1.5.0
rootdir: /llama-stack
configfile: pyproject.toml
plugins: asyncio-0.24.0, anyio-4.6.2.post1
asyncio: mode=Mode.STRICT, default_loop_scope=None
collected 18 items / 15 deselected / 3 selected

llama_stack/providers/tests/memory/test_memory.py ...                                                                                                                                                                     [100%]

========================================================================================= 3 passed, 15 deselected, 6 warnings in 0.03s ==========================================================================================
```

llama_stack/providers/tests/memory/fakes.py

@@ -0,0 +1,92 @@
+# 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 typing import Any, Dict, List, Optional
+
+from llama_stack.apis.memory.memory import (
+    Memory,
+    MemoryBankDocument,
+    MemoryBankStore,
+    QueryDocumentsResponse,
+)
+from llama_stack.apis.memory_banks.memory_banks import KeyValueMemoryBank
+from llama_models.llama3.api.datatypes import *  # noqa: F403
+from llama_stack.distribution.datatypes import *  # noqa: F403
+
+
+# MemoryBanks test fake implementation to
+# support behaviors tested in test_memory.py
+class MemoryBanksTestFakeImpl(MemoryBanks):
+    def __init__(self):
+        self.memory_banks: Dict[str, MemoryBank] = dict()
+
+    async def list_memory_banks(self) -> List[MemoryBank]:
+        return list(self.memory_banks.values())
+
+    async def get_memory_bank(self, memory_bank_id: str) -> Optional[MemoryBank]:
+        if memory_bank_id in self.memory_banks:
+            return self.memory_banks[memory_bank_id]
+
+    async def register_memory_bank(
+        self,
+        memory_bank_id: str,
+        params: BankParams,
+        provider_id: Optional[str] = None,
+        provider_memory_bank_id: Optional[str] = None,
+    ) -> MemoryBank:
+        memory_bank = KeyValueMemoryBank(
+            identifier=memory_bank_id,
+            provider_id="test::test-fake",
+        )
+        self.memory_banks[memory_bank_id] = memory_bank
+        return self.memory_banks[memory_bank_id]
+
+    async def unregister_memory_bank(self, memory_bank_id: str) -> None:
+        if memory_bank_id in self.memory_banks:
+            del self.memory_banks[memory_bank_id]
+
+
+# Memory test fake implementation to
+# support behaviors tested in test_memory.py
+class MemoryTestFakeImpl(Memory):
+    memory_bank_store: MemoryBankStore
+
+    def __init__(self):
+        self.memory_banks = None
+        self.stubs: Dict[str, Any] = {}
+
+    def set_memory_banks(self, memory_banks: MemoryBanks) -> None:
+        self.memory_banks = memory_banks
+
+    def set_stubs(self, method: str, stubs: Dict[str, Any]):
+        self.stubs[method] = stubs
+
+    async def insert_documents(
+        self,
+        bank_id: str,
+        documents: List[MemoryBankDocument],
+        ttl_seconds: Optional[int] = None,
+    ) -> None:
+        if not await self.memory_banks.get_memory_bank(bank_id):
+            raise ValueError(f"Bank {bank_id} not found")
+        # No-op
+        # We will just ignore documents here since we will init this
+        # test fake with stubs to match expecting query-response pairs
+
+    async def query_documents(
+        self,
+        bank_id: str,
+        query: InterleavedTextMedia,
+        params: Optional[Dict[str, Any]] = None,
+    ) -> QueryDocumentsResponse:
+        if not await self.memory_banks.get_memory_bank(bank_id):
+            raise ValueError(f"Bank {bank_id} not found")
+        if query not in self.stubs["query_documents"]:
+            raise ValueError(
+                f"Stub not created for query {query}, please check your test setup."
+            )
+
+        return self.stubs["query_documents"][query]

llama_stack/providers/tests/memory/fixtures.py

@@ -10,14 +10,62 @@ import tempfile
 import pytest
 import pytest_asyncio
 
+from llama_stack.apis.memory.memory import Chunk, QueryDocumentsResponse
 from llama_stack.distribution.datatypes import Api, Provider, RemoteProviderConfig
+from llama_stack.providers.datatypes import TestFakeProviderConfig
 from llama_stack.providers.inline.memory.faiss import FaissImplConfig
 from llama_stack.providers.remote.memory.pgvector import PGVectorConfig
 from llama_stack.providers.remote.memory.weaviate import WeaviateConfig
 from llama_stack.providers.tests.resolver import construct_stack_for_test
 from llama_stack.providers.utils.kvstore import SqliteKVStoreConfig
-from ..conftest import ProviderFixture, remote_stack_fixture
+from ..conftest import ProviderFixture, remote_stack_fixture, test_fake_stack_fixture
 from ..env import get_env_or_fail
+from .fakes import MemoryBanksTestFakeImpl, MemoryTestFakeImpl
+
+
+@pytest.fixture(scope="session")
+def query_documents_stubs():
+    # These are stubs for the method calls against MemoryTestFakeImpl fake
+    # so the tests inside test_memory will as with a real provider
+    return {
+        "programming language": QueryDocumentsResponse(
+            chunks=[Chunk(content="Python", token_count=1, document_id="")],
+            scores=[0.1],
+        ),
+        "AI and brain-inspired computing": QueryDocumentsResponse(
+            chunks=[Chunk(content="neural networks", token_count=2, document_id="")],
+            scores=[0.1],
+        ),
+        "computer": QueryDocumentsResponse(
+            chunks=[
+                Chunk(content="test-chunk-1", token_count=1, document_id=""),
+                Chunk(content="test-chunk-2", token_count=1, document_id=""),
+            ],
+            scores=[0.1, 0.5],
+        ),
+        "quantum computing": QueryDocumentsResponse(
+            chunks=[Chunk(content="Python", token_count=1, document_id="")],
+            scores=[0.5],
+        ),
+    }
+
+
+@pytest.fixture(scope="session")
+def memory_test_fake(query_documents_stubs) -> ProviderFixture:
+    # Prepare impl instances here, initiate fake objects and set up stubs
+    memory_banks_impl = MemoryBanksTestFakeImpl()
+    memory_impl = MemoryTestFakeImpl()
+    memory_impl.set_memory_banks(memory_banks_impl)
+    memory_impl.set_stubs("query_documents", query_documents_stubs)
+
+    config = TestFakeProviderConfig(
+        impls={
+            Api.memory: memory_impl,
+            Api.memory_banks: memory_banks_impl,
+        }
+    )
+
+    return test_fake_stack_fixture(config)
 
 
 @pytest.fixture(scope="session")
@@ -93,7 +141,7 @@ def memory_chroma() -> ProviderFixture:
     )
 
 
-MEMORY_FIXTURES = ["faiss", "pgvector", "weaviate", "remote", "chroma"]
+MEMORY_FIXTURES = ["test_fake", "faiss", "pgvector", "weaviate", "remote", "chroma"]
 
 
 @pytest_asyncio.fixture(scope="session")