Cleanup: Improve formatting and remove duplicate code in textio.py

[Abhay-raj-srivastava]

🎯 Summary

This PR cleans up textio.py to improve readability, maintainability, and consistency with the rest of the Beam codebase.

✨ Changes Included

• Removed duplicated sections of ReadBuffer and related logic.
• Cleaned up formatting, indentation, and spacing.
• Organized imports and removed unused ones.
• Applied consistent type hints.
• Reduced unnecessary code paths and comments.
• Improved overall code structure without altering any functionality.

🛠 Functional Behavior

This PR introduces no functional changes.
All behavior remains identical — only cleanup and formatting were performed.

📌 Motivation

While exploring Beam's IO code, I noticed redundant sections and inconsistent formatting in textio.py.
This cleanup makes future contributions easier and reduces the complexity of the file.

✔ Tests

No logic was changed, so existing tests should continue to pass.

---

Please review and let me know if any adjustments are needed!

[gemini-code-assist]

Summary of Changes

Hello @Abhay-raj-srivastava, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request delivers a significant cleanup of the textio.py module, focusing on improving its overall readability, maintainability, and consistency with the Apache Beam codebase. The changes involve applying modern Python features like comprehensive type hints and f-strings, organizing imports, and refining code formatting. The primary goal is to simplify future development and reduce complexity without introducing any alterations to the module's functional behavior.

Highlights

• Improved Type Hinting: Extensive type hints have been added across various classes and methods within textio.py, including _TextSource, _TextSourceWithFilename, and _TextSink, enhancing code clarity and maintainability.
• Enhanced Code Formatting: The pull request introduces consistent formatting, indentation, and spacing improvements throughout the textio.py file, making the code more readable and aligned with Beam's style guidelines. This includes updating string formatting to use f-strings where appropriate.
• Streamlined Imports and Docstrings: Import statements have been organized, and unused imports removed. Additionally, docstrings for Pandas-related I/O transforms (ReadFromCsv, WriteToCsv, ReadFromJson, WriteToJson) have been refactored for conciseness, directing users to the underlying Pandas documentation for detailed parameter explanations.
• No Functional Changes: All changes are purely for cleanup and refactoring; no functional behavior of the textio.py module has been altered, ensuring existing pipelines will continue to operate identically.

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[Abhay-raj-srivastava]

Hi maintainers!
This PR is ready for CI. Could you please approve running the precommit workflows?
This PR only contains cleanup and formatting changes in textio.py, with no functional modifications.
Thanks!