fix: unclosed file handle in evaluate_ner() when dumping metrics

## Problem

In `scispacy/train_utils.py` line 29, `evaluate_ner()` opens a file handle for writing metrics but never closes it:

```python
json.dump(metrics, open(dump_path, "a+"))
```

The `open()` call creates a file object that is passed directly to `json.dump()`. After `json.dump()` returns, the file handle is not explicitly closed. While CPython"s reference counting will eventually close it, this is not guaranteed on other Python implementations (PyPy, Jython) and is a resource leak.

More importantly, without explicit close/flush, the written data may not be flushed to disk immediately, which could cause data loss if the process crashes after this point.

## Affected File
- `scispacy/train_utils.py` line 29