fix: Setting default value for metadata_token_count in case the key is not found (#2199)

# What does this PR do?
If a user has previously serialized data into their vector store without
the `metadata_token_count` in the chunk, the `query` method will fail in
a server error. This fixes that edge case by returning 0 when the key is
not detected. This solution is suboptimal but I think it's better to
understate the token size rather than recalculate it and add unnecessary
complexity to the retrieval code.

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan
[Describe the tests you ran to verify your changes with result
summaries. *Provide clear instructions so the plan can be easily
re-executed.*]

[//]: # (## Documentation)

Signed-off-by: Francisco Javier Arceo <[email protected]>

Author: franciscojavierarceo

llama_stack/providers/inline/tool_runtime/rag/memory.py

@@ -146,7 +146,7 @@ async def query(
         for i, chunk in enumerate(chunks):
             metadata = chunk.metadata
             tokens += metadata["token_count"]
-            tokens += metadata["metadata_token_count"]
+            tokens += metadata.get("metadata_token_count", 0)
 
             if tokens > query_config.max_tokens_in_context:
                 log.error(