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/resolver.py

@@ -4,8 +4,10 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
+import importlib.util
 import json
 import tempfile
+
 from typing import Any, Dict, List, Optional
 
 from llama_stack.distribution.datatypes import *  # noqa: F403
@@ -51,8 +53,11 @@ async def construct_stack_for_test(
     try:
         remote_config = remote_provider_config(run_config)
         if not remote_config:
-            # TODO: add to provider registry by creating interesting mocks or fakes
-            impls = await construct_stack(run_config, get_provider_registry())
+            # Here we create instance of registry with optional fake provider
+            provider_registry = setup_provider_registry_for_test(
+                run_config, get_provider_registry()
+            )
+            impls = await construct_stack(run_config, provider_registry)
         else:
             # we don't register resources for a remote stack as part of the fixture setup
             # because the stack is already "up". if a test needs to register resources, it
@@ -73,6 +78,60 @@ async def construct_stack_for_test(
     return test_stack
 
 
+def setup_provider_registry_for_test(
+    run_config: StackRunConfig, provider_registry: Dict[Api, Dict[str, ProviderSpec]]
+) -> Dict[Api, Dict[str, ProviderSpec]]:
+    provider_registry = get_provider_registry()
+    for api_name, providers in run_config.providers.items():
+        for provider in providers:
+            if provider.provider_type == "test::fake":
+                # Check if the fake provider module exists for the API that is trying
+                # to use test::fake provider_type
+                provider_fake_module_name = (
+                    f"llama_stack.providers.tests.{api_name}.fakes"
+                )
+                provider_fake_module_spec = importlib.util.find_spec(
+                    provider_fake_module_name
+                )
+                if provider_fake_module_spec is None:
+                    raise ValueError(
+                        f"Fake provider module {provider_fake_module_name} does not exist. "
+                        f"The module must be defined inside the providers/tests/{api_name}/fakes.py file."
+                    )
+
+                # Import the module so we can validate that the config class exists
+                provider_fake_module = importlib.import_module(
+                    provider_fake_module_name
+                )
+
+                # Check if the fake provider config class exists
+                # The class name is derived from the provider type e.g.
+                #   provider_id: "example_provider" -> class_name: "ExampleProviderConfig"
+                provider_fake_config_class_name = (
+                    f"{provider.provider_id}_config".title().replace("_", "")
+                )
+                if not hasattr(provider_fake_module, provider_fake_config_class_name):
+                    raise ValueError(
+                        f"Fake provider config class {provider_fake_config_class_name} "
+                        f"does not exist in module {provider_fake_module_name}. "
+                        f"The config class must be defined inside the providers/tests/{api_name}/fakes.py file."
+                    )
+
+                provider_fake_config_class_path = f"llama_stack.providers.tests.{api_name}.fakes.{provider_fake_config_class_name}"
+
+                api = getattr(Api, api_name)
+                fake_api_spec = InlineProviderSpec(
+                    api=api,
+                    provider_type="test::fake",
+                    pip_packages=[],
+                    module=provider_fake_module_name,
+                    config_class=provider_fake_config_class_path,
+                )
+                provider_registry[api].update({"test::fake": fake_api_spec})
+
+    return provider_registry
+
+
 def remote_provider_config(
     run_config: StackRunConfig,
 ) -> Optional[RemoteProviderConfig]: