Test and unify text splitter functionality

[dayesouza]

Description

The changes involve refactoring the text splitting methods, adding new helper functions, and updating the corresponding unit tests.

Proposed Changes

• Replaced the _split_text_on_tokens function with split_multiple_texts_on_tokens to improve control over the chunking process.
• Refactored the split_text method to handle both single and multiple texts, and added the split_single_text_on_tokens and split_multiple_texts_on_tokens functions for better text splitting
• Added comprehensive unit tests for the new text splitting functions, including tests for edge cases and large inputs.

Checklist

• I have tested these changes locally.
• I have reviewed the code changes.
• I have added appropriate unit tests.