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

[Matt711]

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

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.