Topic deletion fails if topics have custom lables

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### Have you searched existing issues? 🔎

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### Describe the bug

If the model has custom labels set, topic deletion fails with error:

```
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
Cell In[10], line 1
----> 1 topic_model.delete_topics([2])

File ~/miniconda3/envs/tgram/lib/python3.13/site-packages/bertopic/_bertopic.py:2275, in BERTopic.delete_topics(self, topics_to_delete)
2271 # Update custom labels if they exist
2272 if hasattr(self, "custom_labels_") and self.custom_labels_ is not None:
2273 new_labels = {
2274 (final_mapping[old_topic] if old_topic != -1 else -1): label
-> 2275 for old_topic, label in self.custom_labels_.items()
2276 if old_topic not in topics_to_delete
2277 }
2278 self.custom_labels_ = new_labels
2280 # Update topic representations

AttributeError: 'list' object has no attribute 'items'
```

The delete topics function expects custom_labels_ to be a dictionary, but its a list, I can see in other places in the code it does check for the object type (list or dict) and handles it accordingly, but not here.

### Reproduction

```python
from bertopic import BERTopic

from sklearn.datasets import fetch_20newsgroups
docs = fetch_20newsgroups(subset='all', remove=('headers', 'footers', 'quotes'))['data']

topic_model = BERTopic()
topics, probs = topic_model.fit_transform(docs)

topic_model.delete_topics([2]) # works fine

#set custom labels
topic_labels = topic_model.generate_topic_labels(nr_words=3, topic_prefix=True,word_length=8,separator=" - ")
topic_model.set_topic_labels(topic_labels)

topic_model.delete_topics([2]) # deletion fails with the error shown above
```

### BERTopic Version

0.17.4

Start in bertopic/_bertopic.py at the delete_topics method around line 2275, then compare how other code handles custom_labels_ as a list or dictionary. Reproduce the failure with the provided BERTopic example and verify that deleting a topic after set_topic_labels succeeds for list-based labels.

Written by the indexing model from the issue text.