Testing - Memory provider fakes

Summary:
Implementing Memory provider fakes as discussed in this draft https://github.com/meta-llama/llama-stack/pull/490#issuecomment-2492877393.

High level changes:
* Fake provider is specified via the "fake" mark
* Test config will setup a fake fixture for the run of the test
* Test resolver checks fixtures and upon finding a fake provider it injects InlineProviderSpec for fake provider
* Fake provider gets resolved through path/naming convention
* Fake provider implementaion is contained to the tests/ directory and implements stubs and method fakes with minimal functionality to simulate real provider

Instructins to creating a fake
* Create the "fakes" module inside the provider test directory
* Inside the module implement `get_provider_impl` that will return fake implementation object
* Name the fake implementation class to match the fake provider id (e.g. memory_fake -> MemoryFakeImpl)
  * Same rule for the config (e.g. memory_fake -> MemoryFakeConfig)
* Add fake fixture (in the fixtures.py) and setup methods stubs there

Test Plan:
Run memory tests
```
pytest llama_stack/providers/tests/memory/test_memory.py -m "fake" -v -s --tb=short

====================================================================================================== test session starts ======================================================================================================
platform darwin -- Python 3.11.10, pytest-8.3.3, pluggy-1.5.0 -- /opt/homebrew/Caskroom/miniconda/base/envs/llama-stack/bin/python
cachedir: .pytest_cache
rootdir: /Users/vivic/Code/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::TestMemory::test_banks_list[fake] PASSED
llama_stack/providers/tests/memory/test_memory.py::TestMemory::test_banks_register[fake] PASSED
llama_stack/providers/tests/memory/test_memory.py::TestMemory::test_query_documents[fake] The scores are: [0.5]
PASSED

========================================================================================= 3 passed, 15 deselected, 10 warnings in 0.46s =========================================================================================

```

llama_stack/providers/tests/memory/fakes.py

@@ -0,0 +1,84 @@
+# 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_models.schema_utils import json_schema_type
+from pydantic import BaseModel
+
+from llama_stack.apis.memory import *  # noqa: F403
+from llama_stack.apis.memory_banks import MemoryBank
+from llama_stack.providers.datatypes import MemoryBanksProtocolPrivate
+
+
+@json_schema_type
+class InlineMemoryFakeConfig(BaseModel):
+    pass
+
+
+class InlineMemoryFakeImpl(Memory, MemoryBanksProtocolPrivate):
+    method_stubs: Dict[str, Any] = {}
+    memory_banks: Dict[str, MemoryBank] = {}
+
+    @staticmethod
+    def stub_method(method_name: str, return_value_matchers: Dict[str, Any]) -> None:
+        if method_name in InlineMemoryFakeImpl.method_stubs:
+            InlineMemoryFakeImpl.method_stubs[method_name].update(return_value_matchers)
+            return
+        InlineMemoryFakeImpl.method_stubs[method_name] = return_value_matchers
+
+    def __init__(self, config: InlineMemoryFakeConfig) -> None:
+        pass
+
+    async def initialize(self) -> None:
+        pass
+
+    async def shutdown(self) -> None:
+        pass
+
+    async def register_memory_bank(
+        self,
+        memory_bank: MemoryBank,
+    ) -> None:
+        InlineMemoryFakeImpl.memory_banks[memory_bank.memory_bank_id] = memory_bank
+
+    async def list_memory_banks(self) -> List[MemoryBank]:
+        return list(InlineMemoryFakeImpl.memory_banks.values())
+
+    async def unregister_memory_bank(self, memory_bank_id: str) -> None:
+        if memory_bank_id not in InlineMemoryFakeImpl.memory_banks:
+            raise ValueError(f"Bank {memory_bank_id} not found.")
+        del InlineMemoryFakeImpl.memory_banks[memory_bank_id]
+
+    async def insert_documents(
+        self,
+        bank_id: str,
+        documents: List[MemoryBankDocument],
+        ttl_seconds: Optional[int] = None,
+    ) -> None:
+        pass
+
+    async def query_documents(
+        self,
+        bank_id: str,
+        query: InterleavedTextMedia,
+        params: Optional[Dict[str, Any]] = None,
+    ) -> QueryDocumentsResponse:
+        if query in InlineMemoryFakeImpl.method_stubs["query_documents"]:
+            return InlineMemoryFakeImpl.method_stubs["query_documents"][query]
+        raise ValueError(
+            f"Stub for query '{query}' not found, please set up expected result"
+        )
+
+
+async def get_provider_impl(config: InlineMemoryFakeConfig, _deps: Any):
+    assert isinstance(
+        config, InlineMemoryFakeConfig
+    ), f"Unexpected config type: {type(config)}"
+
+    impl = InlineMemoryFakeImpl(config)
+    await impl.initialize()
+    return impl