Move Row to typehints/row.py, add compatibility alias in pvalue.py, update imports, and update CHANGES.md

[nikitagrover19]

Move Row to typehints/row.py, add compatibility alias in pvalue.py, update imports, and update CHANGES.md

This change reorganizes the Python SDK by moving the Row class into a dedicated module under apache_beam.typehints.
The goal is to improve module structure, reduce coupling, and better align Beam’s schema utilities with the type-hinting subsystem.

Description

This PR moves the Row implementation from apache_beam.pvalue into a new file, apache_beam.typehints.row.
This is a more appropriate location for a schema-aware, type-hinted data container and helps simplify the structure of pvalue.py.

To maintain full backward compatibility, pvalue.py now exposes a compatibility alias so that existing user code importing apache_beam.Row continues to work without modification.

Additional Updates

• Updated apache_beam/init.py to re-export Row.
• Updated imports wherever the previous module path was used.
• Added an entry to CHANGES.md documenting the relocation.

This improves code organization and moves schema utilities closer to the rest of Beam’s type-hinting system, without breaking any public APIs.

Fixes #35095

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Summary of Changes

Hello @nikitagrover19, 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 refactors the Python SDK by relocating the Row class to a more semantically appropriate module within the type-hinting subsystem. This change enhances code organization and reduces coupling, while meticulously preserving backward compatibility for users. The move integrates schema-aware data containers more closely with Beam's type-hinting capabilities, ensuring a cleaner and more logical structure without impacting existing public APIs.

Highlights

• Module Reorganization: The Row class has been moved from apache_beam.pvalue to a new, dedicated module: apache_beam.typehints.row. This aims to improve the module structure and better align schema utilities with the type-hinting subsystem.
• Backward Compatibility: A compatibility alias for Row has been added to pvalue.py to ensure that existing user code importing apache_beam.Row continues to function without modification.
• Import Updates: All necessary import statements across the codebase, including in apache_beam/__init__.py, have been updated to reflect the new location of the Row class.
• Changelog Documentation: The CHANGES.md file has been updated to document the relocation of the Row class and the introduction of the compatibility alias.

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