fix(traverse_util): validate keys in flatten_dict when separator is specified (#5554)

[Ultron09]

Summary

Fixes #5554

When flax.traverse_util.flatten_dict is called with a separator (sep is not None):

1. Case 1 (Structural corruption): A string key containing the separator (e.g. {"a/b": 1, "c": {"d": 2}} with sep="/") flattens to {"a/b": 1, "c/d": 2}. When unflatten_dict(..., sep="/") is called, "a/b" is split into a nested path {"a": {"b": 1}}, silently corrupting the round-trip structure without error.
2. Case 2 (Opaque TypeError): A non-string key (e.g. {1: {2: 3}} with sep="/") surfaces an internal TypeError: sequence item 0: expected str instance, int found from deep inside string joining.

Solution

• Validates each key in _flatten when sep is not None to ensure it is a string and does not contain sep.
• Raises a descriptive ValueError:

  ValueError: flatten_dict with sep='/' requires string keys that do not contain the separator; got key 'a/b' at path ('a/b',)
  

• Updates flatten_dict docstrings to document the string/separator requirement and ValueError.
• Added unit tests in traverse_util_test.py covering non-string keys, separator-containing keys, nested dictionary paths, and verifying sep=None behavior remains unaffected.

cc @IvyZX @cgarciae @jheek @Amey-Thakur