fix: preserve exception cause

[fernandocorreia-galileo]

Fixes #880

Problem:

When the retry mechanism re-raises non-retryable exceptions, it explicitly clears the exception chain by using raise final_exc from None. This breaks explicit exception chaining created with raise ... from ..., making the original cause (cause) inaccessible for debugging.

Fix:

Modified _default_exception_factory() in retry_base.py to preserve the __cause__ attribute when re-raising non-retryable exceptions. Instead of returning (exc_list[-1], None), we now return (exc_list[-1], getattr(exc_list[-1], '__cause__', None)).

This ensures that exception chains are maintained through the retry mechanism, preserving debugging information and meeting developer expectations for exception handling.

Testing:

• Added comprehensive test coverage in test_exception_chain_preservation.py
• All existing retry tests continue to pass
• Verified both simple and nested exception chains are preserved

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Fixes #<issue_number_goes_here> 🦕

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[gemini-code-assist]

Summary of Changes

Hello @fernandocorreia-galileo, 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 resolves an issue where the retry mechanism would inadvertently clear the "cause" attribute of exceptions, breaking explicit exception chaining. By ensuring that the original cause is preserved, the change significantly improves the debuggability and traceability of errors that occur within retryable operations, providing clearer context for why an operation ultimately failed.

Highlights

• Exception Chaining Preservation: The retry mechanism now correctly preserves the "cause" attribute of exceptions, ensuring that explicit exception chaining (raise ... from ...) is maintained when non-retryable errors are re-raised.
• New Test Coverage: A new asynchronous test file has been added to specifically verify the preservation of exception chains, covering both single-level and deeply nested exception scenarios within the retry decorator.

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[gemini-code-assist]

Code Review

This pull request correctly fixes an issue where the exception cause (__cause__) was not being preserved in the retry logic, leading to broken exception chains. The change in google/api_core/retry/retry_base.py ensures that the original cause is carried over when an exception is re-raised. The newly added asynchronous tests in tests/asyncio/retry/test_exception_chain_preservation.py effectively validate this fix for async scenarios.

My review includes a suggestion to simplify the implementation for better readability and a recommendation to add synchronous tests to ensure full coverage of the fix, as the change impacts both sync and async retry mechanisms.

[gemini-code-assist]

These tests are great for verifying the fix for async retries. Since the underlying change in retry_base.py also affects synchronous retries, please consider adding a parallel set of tests for the synchronous case to ensure complete coverage.