Add payload validation error factory

[bergundy]

Summary

• export temporalio.converter.create_payload_validation_error
• create a non-retryable PayloadValidationError ApplicationError with structured details
• replace positive Nexus test constructions with the public factory while retaining retryable compatibility coverage

Testing

• poe format
• focused converter factory and Nexus mapper tests
• targeted Ruff, Pyright, mypy, basedpyright, and pydocstyle checks

Full optional-suite collection requires AWS and Google ADK dependencies that are not installed in this checkout.

[VegetarianOrc]

Looks good to me! Just a small question about a test.