[BUG] "When using a chatbot with a dictionary set to large chunks (e.g., 4096), an error occurs after asking questions two or more times

## Describe the bug

When using a chatbot with a dictionary set to large chunks (e.g., 4096) , an error occurs after asking questions two or more times.

## To Reproduce

Steps to reproduce the behavior:

1. Create Chatbot
large chunks (e.g., 4096) & Contextual Grounding threshold is set to 0
2. asking questions two or more
3. See error

## Screenshots
1. Chatbot Setting

3.

## Additional context
・Confirmed conditions:
　　 Reproduced with hierarchical chunks
　　　 Parent Chunksize=4096, Child Chunksize=256 - Error occurs mostly on second query
　　　 Parent Chunksize=2048, Child Chunksize=256 - Error occurs mostly on third query
　　　Parent Chunksize=1500, Child Chunksize=300 - Error occurs after fourth query
　　 With fixed chunks
　　　　Parent Chunksize=4096　Error occurs mostly on second query
　　　 Parent Chunksize=2048　 Error occurs mostly on third query

・Similar to #637, this error occurs even when the Harmful Categories threshold is set to 0

Reproduce the chatbot failure using hierarchical and fixed chunks with the listed parent and child sizes, a Contextual Grounding threshold of 0, and repeated questions. Compare the behavior with issue #637, then trace the error from the second, third, or fourth query; done means repeated questions no longer produce the reported error under these settings.

Written by the indexing model from the issue text.