test(spanner): fix IndexError in async session retry deadline unit test (#17140)

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### Root Cause
In Python 3.13 (and under recent `pytest-asyncio` / `asyncio` runtimes),
the asynchronous event loop and task scheduling infrastructure makes
internal calls to `time.time()` during coroutine execution and retry
delay handling.

In `test_run_in_transaction_w_abort_w_retry_metadata_deadline`,
`time.time` was mocked using a helper that popped from a fixed 2-element
list on every invocation (`_results.pop(0)`). The extra internal event
loop calls exhausted `_results` prematurely, deterministically raising
an `IndexError: pop from empty list` when `_delay_until_retry` checked
the deadline.

### Solution
Updated the `_time` mock helper in
`packages/google-cloud-spanner/tests/unit/_async/test_session.py` to
return the last timestamp repeatedly once `_results` has reached its
final element:
```python
def _time(_results=[1, 1.5]):
    if len(_results) > 1:
        return _results.pop(0)
    return _results[0]

Author: sakthivelmanii

packages/google-cloud-spanner/tests/unit/_async/test_session.py

@@ -1802,7 +1802,9 @@ async def unit_of_work(txn, *args, **kw):
 
         # retry once w/ timeout_secs=1
         def _time(_results=[1, 1.5]):
-            return _results.pop(0)
+            if len(_results) > 1:
+                return _results.pop(0)
+            return _results[0]
 
         with mock.patch("time.time", _time):
             with mock.patch(