Add boundary tests for text buffer

[chengyitang]

This PR adds test coverage to ensure correct behavior in the following situations:

• Handling very long single lines (10,000 characters)
• Handling documents with a large number of lines (1,000 lines)

These tests help prevent potential issues that might appear with future code changes for TextBuffer.

[github-actions]

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[chengyitang]

I have read the CLA Document and I hereby sign the CLA