The method to_netcdf does not preserve chunks

What happened?

Methods to_zarr and to_netcdf behave inconsistently for chunked dataset. The latter does not preserve existing chunk information, the chunks must be specified within the encoding dictionary.

What did you expect to happen?

I expected the behaviour to be consistent for for all to_XXX() methods.

Minimal Complete Verifiable Example

import xarray as xr
import dask.array as da

rng = da.random.RandomState()
shape = (20, 20)
chunks = [10, 10]
dims = ["x", "y"]
z = rng.standard_normal(shape, chunks=chunks)
ds = xr.DataArray(z, dims=dims, name="z").to_dataset()
ds.chunks
# This one is rechunked
ds.to_netcdf("/tmp/test1.nc", encoding={"z": {"chunksizes": (5, 5)}})
# This one is not rechunked, also original chunks are lost
ds.chunk({"x": 5, "y": 5}).to_netcdf("/tmp/test2.nc")
# This one is rechunked
ds.chunk({"x": 5, "y": 5}).to_zarr("/tmp/test2", mode="w")

Frozen({'x': (10, 10), 'y': (10, 10)})
<xarray.backends.zarr.ZarrStore at 0x7f3669f1af80>

xr.open_mfdataset("/tmp/test1.nc").chunks
xr.open_mfdataset("/tmp/test2.nc").chunks
xr.open_mfdataset("/tmp/test2", engine="zarr").chunks

Frozen({'x': (5, 5, 5, 5), 'y': (5, 5, 5, 5)})
Frozen({'x': (20,), 'y': (20,)})
Frozen({'x': (5, 5, 5, 5), 'y': (5, 5, 5, 5)})

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.
• Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

No response

Anything else we need to know?

I did get the same results for h5netcdf and scipy backends, so I am not sure whether this is a bug or not.
The above code is a modified version of #2198.
A suggestion: the documentation provides only examples of encoding styles. It would be helpful to provide links to a full specification.

Environment

Start by running the provided minimal example and comparing the chunk handling in to_netcdf with to_zarr across the reported backends. Trace the existing chunk and encoding behavior in the relevant xarray IO paths. Done means to_netcdf preserves existing dataset chunks consistently with the other to_XXX methods, while explicit encoding chunks still work.