Fix model_input_names singleton issue causing shared state

Fixes huggingface#42024

The model_input_names attribute was defined as a class-level list, and
when initializing tokenizer instances, they were all pointing to the same
list object. This meant modifying model_input_names on one instance would
affect all other instances.

The issue was in tokenization_utils_base.py line 1417:
```python
self.model_input_names = kwargs.pop("model_input_names", self.model_input_names)
```

When no model_input_names is passed in kwargs, it would use the class
attribute directly (self.model_input_names), creating a reference to the
shared list instead of creating a new list for the instance.

Fixed by wrapping it in list() to ensure each instance gets its own copy:
```python
self.model_input_names = list(kwargs.pop("model_input_names", self.model_input_names))
```

This is a standard pattern for handling mutable default values in Python.

Author: Yashwant Bezawada

src/transformers/tokenization_utils_base.py

@@ -1414,7 +1414,7 @@ def __init__(self, **kwargs):
                 f"Truncation side should be selected between 'right' and 'left', current value: {self.truncation_side}"
             )
 
-        self.model_input_names = kwargs.pop("model_input_names", self.model_input_names)
+        self.model_input_names = list(kwargs.pop("model_input_names", self.model_input_names))
 
         # By default, cleaning tokenization spaces for both fast and slow tokenizers
         self.clean_up_tokenization_spaces = kwargs.pop("clean_up_tokenization_spaces", False)