[Task]: Set beam schema proto field in user_type in typehint/schemas.py

[riteshghorse]

What happened?

Had to revert user_type._beam_schema_proto = _Ephemeral(schema) because of a failure caught in #29684. Add this again to generate correct documentation such that it doesn't affect the DecimalLogicalType in python.

https://github.com/apache/beam/blob/fa47640888cba41ab675b8a4bdb22b1d3a4b1fa1/sdks/python/apache_beam/typehints/schemas.py#L564

Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

Issue Components

• [X] Component: Python SDK
• [ ] Component: Java SDK
• [ ] Component: Go SDK
• [ ] Component: Typescript SDK
• [ ] Component: IO connector
• [ ] Component: Beam YAML
• [ ] Component: Beam examples
• [ ] Component: Beam playground
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[tvalentyn]

tentatively assigning to @robertwb who worked on the original change and has context -- feel free to add details and unassign if you are not planning to work on it. Thank you!