fix: preserve exception cause (#879)

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 a new test case in `tests/unit/retry/test_retry_base.py` that
checks that the cause from the chained exception is preserved by
`build_retry_error`.

Author: fernandocorreia-galileo

google/api_core/retry/retry_base.py

@@ -164,8 +164,10 @@ def build_retry_error(
             src_exc,
         )
     elif exc_list:
-        # return most recent exception encountered
-        return exc_list[-1], None
+        # return most recent exception encountered and its cause
+        final_exc = exc_list[-1]
+        cause = getattr(final_exc, "__cause__", None)
+        return final_exc, cause
     else:
         # no exceptions were given in exc_list. Raise generic RetryError
         return exceptions.RetryError("Unknown error", None), None

tests/unit/retry/test_retry_base.py

@@ -74,6 +74,26 @@ def test_build_retry_error_empty_list():
     assert src.message == "Unknown error"
 
 
+def test_build_retry_error_preserves_cause():
+    """
+    build_retry_error should preserve __cause__ from chained exceptions.
+    """
+    from google.api_core.retry import build_retry_error
+    from google.api_core.retry import RetryFailureReason
+
+    # Create an exception with explicit cause
+    cause = ValueError("root cause")
+    exc = RuntimeError("wrapper")
+    exc.__cause__ = cause
+
+    src, found_cause = build_retry_error(
+        [exc], RetryFailureReason.NON_RETRYABLE_ERROR, None
+    )
+
+    assert src is exc
+    assert found_cause is cause
+
+
 def test_build_retry_error_timeout_message():
     """
     should provide helpful error message when timeout is reached