chore: Add support for vector-stores files api for Milvus (#2582)

# What does this PR do?
### Summary

This pull request implements support for the OpenAI Vector Store Files
API for the Milvus vector store provider in `llama_stack`. It enables
storing, loading, updating, and deleting file metadata and file contents
in Milvus collections, allowing OpenAI vector store files to be managed
directly within Milvus.

### Main Changes

- **Milvus Vector Store Files API Implementation**
- Implements all required methods for storing, loading, updating, and
deleting vector store file metadata and contents
(`_save_openai_vector_store_file`, `_load_openai_vector_store_file`,
`_load_openai_vector_store_file_contents`,
`_update_openai_vector_store_file`,
`_delete_openai_vector_store_file_from_storage`).
- Uses two Milvus collections: `openai_vector_store_files` (for
metadata) and `openai_vector_store_files_contents` (for chunked file
contents).
- Collections are created dynamically if they do not exist, with
appropriate schema definitions.
- **Collection Name Sanitization**
- Adds a `sanitize_collection_name` utility to ensure Milvus collection
names only contain valid characters (letters, numbers, underscores).
- **Testing**
- Updates test skip logic to include `"inline::milvus"` for cases where
the OpenAI Vector Store Files API is not supported, improving
integration test accuracy.
- **Other Improvements**
  - Passes `kvstore` to `MilvusIndex` for consistency.
- Removes obsolete NotImplementedErrors and legacy code for file
storage.

## Test Plan
CI and tested via a test script

## Notes
- `VectorDB` currently uses the `name` as the `identifier` in
`openai_create_vector_store`. We need to add `name` as a field to
`VectorDB` and generate the `identifier` upon creation. OpenAI is not
idempotent with respect to the `name` field that they pass (i.e., you
can pass the same name multiple times and OpenAI will generate a new
identifier). I'll add a follow up PR for this.
- The `Files` api needs to use `files-` as a prefix in the identifier. I
have updated the Vector Store to use the OpenAI prefix `vs_*`.

---------

Signed-off-by: Francisco Javier Arceo <farceo@redhat.com>

tests/integration/vector_io/test_openai_vector_stores.py

@@ -31,7 +31,7 @@ def skip_if_provider_doesnt_support_openai_vector_stores(client_with_models):
 def skip_if_provider_doesnt_support_openai_vector_store_files_api(client_with_models):
     vector_io_providers = [p for p in client_with_models.providers.list() if p.api == "vector_io"]
     for p in vector_io_providers:
-        if p.provider_type in ["inline::faiss", "inline::sqlite-vec"]:
+        if p.provider_type in ["inline::faiss", "inline::sqlite-vec", "inline::milvus"]:
             return
 
     pytest.skip("OpenAI vector stores are not supported by any provider")
@@ -524,7 +524,6 @@ def test_openai_vector_store_attach_files_on_creation(compat_client_with_empty_s
     file_ids = valid_file_ids + [failed_file_id]
     num_failed = len(file_ids) - len(valid_file_ids)
 
-    # Create a vector store
     vector_store = compat_client.vector_stores.create(
         name="test_store",
         file_ids=file_ids,