Fix split file to handle edge case where overlap size > last chunk size

[rocks6]

Background

Currently split_file will double count the last overlap# chars in a document due to the way we are consuming the chunks

Changes

Add logic to handle this case in the split_file function

Documentation

Commented

Test Plan

Tested with new file_operation unit tests, which will be included in a following PR, and tested with a file operation prompt

PR Quality Checklist

• My pull request is atomic and focuses on a single change.
• I have thoroughly tested my changes with multiple different prompts.
• I have considered potential risks and mitigations for my changes.
• I have documented my changes clearly and comprehensively.
• I have not snuck in any "extra" small tweaks changes

[p-i-]

Does this invalidate #2088 ?

[socialmedialabs]

Can't confirm this is working. I just switched to master branch and checked - Auto-GPT failed with the same error when I asked it to read a Python file from my HD:

    raise self.handle_error_response(
openai.error.InvalidRequestError: This model's maximum context length is 8191 tokens, however you requested 216094 tokens (216094 in your prompt; 0 for the completion). Please reduce your prompt; or completion length.

[rocks6]

My PR was related to the fact that the last chunk of a split file would be double counted, that may be a separate issue