`_reset()` reverts unrelated `executor_options` fields to their default

Part of #23740.

**Describe the bug**
`pl.GPUEngine.__init__` does `self.config = kwargs`, a wholesale replacement. `_reset()`
only carries `kvikio_nthreads` and `quent_context` forward (and gets `quent_context`
wrong), every other field is lost on any `_reset()` call that doesn't happen to
re-specify it.

**Steps/Code to reproduce bug**
```python
from cudf_polars.engine.spmd import SPMDEngine
from cudf_polars.utils.config import ConfigOptions

with SPMDEngine(executor_options={"max_rows_per_partition": 500}) as engine:
engine._reset(executor_options={"fallback_mode": "raise"}) # unrelated
cfg = ConfigOptions.from_polars_engine(engine)
print(cfg.executor.max_rows_per_partition)
```

**Expected behavior**
`500`, unchanged. Actual: `1_000_000`, the hardcoded default.