[yaml] : add more error handling tests and website example

[derrickaw]

1. Investigation work for [Feature Request]: YAML - how to output the error of error_handling in sub yaml? #37080.
2. Add more tests to cover this scenario of having a Provider that has error handling.
3. Provide website example of provider with error handling.

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[gemini-code-assist]

Summary of Changes

Hello @derrickaw, 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 significantly improves the robustness of error handling within Apache Beam's YAML transforms. It introduces comprehensive test coverage for scenarios involving multiple transforms generating errors and specifically addresses error handling within custom YAML providers. Furthermore, it provides clear documentation and examples for users to effectively define and consume error outputs from their custom YAML transforms, ensuring better pipeline stability and error visibility.

Highlights

• Enhanced Error Handling Tests: New test cases have been added to validate error handling scenarios, including combining errors from multiple transforms and ensuring proper error output consumption.
• Custom YAML Provider Error Handling: Introduced dedicated tests for custom YAML providers to ensure they correctly expose and manage error outputs, including scenarios where error outputs must be consumed.
• Documentation for Custom Providers: Updated the documentation to include a new section on how to implement and consume error handling within custom YAML transforms, complete with practical examples.

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[derrickaw]

Run Python_Coverage PreCommit 3.10

[derrickaw]

Run Python PreCommit 3.12

[derrickaw]

Run Python PreCommit 3.13

[derrickaw]

Run Python PreCommit 3.13

[derrickaw]

Failures don't seem related.

[derrickaw]

Run Python PreCommit 3.13

[github-actions]

Assigning reviewers:

R: @damccorm for label python.
R: @Abacn for label website.

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[damccorm]

Thanks!