fix(spanner): guard against empty exc.errors in run_in_transaction

[abhi-0203]

Summary

Guard against empty exc.errors in Session.run_in_transaction to prevent IndexError.

Bug: When run_in_transaction catches an Aborted exception with an empty errors list (e.g. manually instantiated Aborted("...")), it crashes with IndexError: list index out of range on exc.errors[0].

Fix: Add a conditional guard: exc.errors[0] if exc.errors else None. When exc.errors is empty, None is passed to _get_retry_delay, which gracefully falls back to default_retry_delay or exponential backoff (2**attempts + random()).

Applied to both occurrences in run_in_transaction (lines 545-547 and 582-584).

Reproducer:

from google.api_core.exceptions import Aborted
from google.cloud import spanner

spanner_client = spanner.Client()
instance = spanner_client.instance("your-instance-id")
database = instance.database("your-database-id")

def callback(transaction):
    raise Aborted("VersionMismatch: CAS failed")

database.run_in_transaction(callback)  # IndexError before fix

Closes #17903

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[gemini-code-assist]

Code Review

This pull request updates the run_in_transaction method in session.py to safely handle cases where an Aborted exception is raised with an empty errors list. By checking if exc.errors is populated before accessing its first element, it prevents potential IndexError exceptions. There are no review comments provided, so I have no additional feedback to offer.