`nnx.cached_partial` errors on jax `Array`s not wrapped in `nnx.Variable`

The Pytree docs say:
> Arrays, Variables, ArrayRefs, and nnx.Pytrees are data.

which seems to imply that assigning raw `Array`s as attributes of `nnx.Pytree`s is supported. For most nnx operations it seems to work fine, but `nnx.cached_partial` fails unless all `Array`s are wrapped in variables:
```python
from flax import nnx
import jax.numpy as jnp

class Mod(nnx.Module):
def __init__(self):
self.a = jnp.arange(3)

def __call__(self, x):
return self.a * x

m = Mod()
print(m(jnp.ones(3)))

m_jit = nnx.jit(m)
print(m_jit(2 * jnp.ones(3)))

@nnx.jit
def f(mod: Mod, x):
return mod(x) + 1

print(f(m, 3 * jnp.ones(3)))

f_partial = nnx.cached_partial(f, m)
print(f_partial(4 * jnp.ones(3)))
```
outputs
```
[0. 1. 2.]
[0. 2. 4.]
[1. 4. 7.]
...
File python3.13/site-packages/flax/nnx/graph.py:1796, in SplitContext.flatten(self, node, with_paths, *filters)
1793 else:
1794 paths = None
1795 leaves = [
-> 1796 variable.get_raw_value() for variable in node_static_cache.variables
1797 ]
1798 else:
1799 graphdef, flat_state = flatten(
1800 node,
1801 ref_index=self.ref_index,
1802 ref_outer_index=ref_outer_index,
1803 with_paths=with_paths,
1804 )

AttributeError: 'jaxlib._jax.ArrayImpl' object has no attribute 'get_raw_value'
```
If I wrap it in an `nnx.Variable` everything works fine:
```
class Mod(nnx.Module):
def __init__(self):
self.a = nnx.Variable(jnp.arange(3))
# ...
```
gives
```
[0. 1. 2.]
[0. 2. 4.]
[1. 4. 7.]
[1. 5. 9.]
```

Is this expected behavior, and is it bad practice/unsupported to have any data in the Pytree that is not wrapped in nnx.Variable? (In the real world, I ran into this issue with a `@flax.struct.dataclass` that had data `Array` fields. I registered the whole dataclass with `nnx.register_data_type` and had assigned it as an attribute to some `nnx.Module`s, getting the same error.)