fix: Agent uses the first configured vector_db_id when documents are provided (#1276)

# What does this PR do?
The agent API allows to query multiple DBs using the `vector_db_ids`
argument of the `rag` tool:
```py
        toolgroups=[
            {
                "name": "builtin::rag",
                "args": {"vector_db_ids": [vector_db_id]},
            }
        ],
```
This means that multiple DBs can be used to compose an aggregated
context by executing the query on each of them.

When documents are passed to the next agent turn, there is no explicit
way to configure the vector DB where the embeddings will be ingested. In
such cases, we can assume that:
- if any `vector_db_ids` is given, we use the first one (it probably
makes sense to assume that it's the only one in the list, otherwise we
should loop on all the given DBs to have a consistent ingestion)
- if no `vector_db_ids` is given, we can use the current logic to
generate a default DB using the default provider. If multiple providers
are defined, the API will fail as expected: the user has to provide
details on where to ingest the documents.

(Closes #1270)

## Test Plan
The issue description details how to replicate the problem.

[//]: # (## Documentation)

---------

Signed-off-by: Daniele Martinoli <dmartino@redhat.com>

llama_stack/distribution/routers/routing_tables.py

@@ -309,13 +309,14 @@ class VectorDBsRoutingTable(CommonRoutingTableImpl, VectorDBs):
         if provider_vector_db_id is None:
             provider_vector_db_id = vector_db_id
         if provider_id is None:
-            # If provider_id not specified, use the only provider if it supports this shield type
-            if len(self.impls_by_provider_id) == 1:
+            if len(self.impls_by_provider_id) > 0:
                 provider_id = list(self.impls_by_provider_id.keys())[0]
+                if len(self.impls_by_provider_id) > 1:
+                    logger.warning(
+                        f"No provider specified and multiple providers available. Arbitrarily selected the first provider {provider_id}."
+                    )
             else:
-                raise ValueError(
-                    "No provider specified and multiple providers available. Please specify a provider_id."
-                )
+                raise ValueError("No provider available. Please configure a vector_io provider.")
         model = await self.get_object_by_identifier("model", embedding_model)
         if model is None:
             raise ValueError(f"Model {embedding_model} not found")