[memory refactor][2/n] Update faiss and make it pass tests (#830)

See https://github.com/meta-llama/llama-stack/issues/827 for the broader
design.

Second part:

- updates routing table / router code 
- updates the faiss implementation


## Test Plan

```
pytest -s -v -k sentence test_vector_io.py --env EMBEDDING_DIMENSION=384
```

llama_stack/providers/tests/vector_io/conftest.py

@@ -0,0 +1,96 @@
+# 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.
+
+import pytest
+
+from ..conftest import (
+    get_provider_fixture_overrides,
+    get_provider_fixture_overrides_from_test_config,
+    get_test_config_for_api,
+)
+
+from ..inference.fixtures import INFERENCE_FIXTURES
+from .fixtures import VECTOR_IO_FIXTURES
+
+
+DEFAULT_PROVIDER_COMBINATIONS = [
+    pytest.param(
+        {
+            "inference": "sentence_transformers",
+            "vector_io": "faiss",
+        },
+        id="sentence_transformers",
+        marks=pytest.mark.sentence_transformers,
+    ),
+    pytest.param(
+        {
+            "inference": "ollama",
+            "vector_io": "faiss",
+        },
+        id="ollama",
+        marks=pytest.mark.ollama,
+    ),
+    pytest.param(
+        {
+            "inference": "sentence_transformers",
+            "vector_io": "chroma",
+        },
+        id="chroma",
+        marks=pytest.mark.chroma,
+    ),
+    pytest.param(
+        {
+            "inference": "bedrock",
+            "vector_io": "qdrant",
+        },
+        id="qdrant",
+        marks=pytest.mark.qdrant,
+    ),
+    pytest.param(
+        {
+            "inference": "fireworks",
+            "vector_io": "weaviate",
+        },
+        id="weaviate",
+        marks=pytest.mark.weaviate,
+    ),
+]
+
+
+def pytest_configure(config):
+    for fixture_name in VECTOR_IO_FIXTURES:
+        config.addinivalue_line(
+            "markers",
+            f"{fixture_name}: marks tests as {fixture_name} specific",
+        )
+
+
+def pytest_generate_tests(metafunc):
+    test_config = get_test_config_for_api(metafunc.config, "vector_io")
+    if "embedding_model" in metafunc.fixturenames:
+        model = getattr(test_config, "embedding_model", None)
+        # Fall back to the default if not specified by the config file
+        model = model or metafunc.config.getoption("--embedding-model")
+        if model:
+            params = [pytest.param(model, id="")]
+        else:
+            params = [pytest.param("all-MiniLM-L6-v2", id="")]
+
+        metafunc.parametrize("embedding_model", params, indirect=True)
+
+    if "vector_io_stack" in metafunc.fixturenames:
+        available_fixtures = {
+            "inference": INFERENCE_FIXTURES,
+            "vector_io": VECTOR_IO_FIXTURES,
+        }
+        combinations = (
+            get_provider_fixture_overrides_from_test_config(
+                metafunc.config, "vector_io", DEFAULT_PROVIDER_COMBINATIONS
+            )
+            or get_provider_fixture_overrides(metafunc.config, available_fixtures)
+            or DEFAULT_PROVIDER_COMBINATIONS
+        )
+        metafunc.parametrize("vector_io_stack", combinations, indirect=True)