fix(nnx): make ToLinen hashable for JAX JIT (#4156)

[Ultron09]

What does this PR do?

Fixes #4156

Standard Linen modules are hashable dataclasses, allowing them to be passed as static arguments to jax.jit functions or used in caches. However, flax.nnx.bridge.ToLinen could fail with TypeError: unhashable type: 'dict' or unhashable sequence errors if instantiated with mutable args (e.g. list) or kwargs (e.g. standard dict).

This PR:

1. Adds __post_init__ in ToLinen to ensure self.args is converted to a tuple and self.kwargs is converted to a flax.core.FrozenDict.
2. Adds a comprehensive regression test test_tolinen_hashable in tests/nnx/bridge/wrappers_test.py covering direct ToLinen, to_linen, and dynamic to_linen_class modules.

Checklist

• This PR fixes a minor issue (e.g.: typo or small bug) or improves the docs (you can dismiss the other checks if that's the case).
• This change is discussed in a Github issue/discussion.
• The documentation and docstrings adhere to the documentation guidelines.
• This change includes necessary high-coverage tests. (No quality testing = no merge!)

cc @cgarciae @vfdev-5 for review.