Fix delete_topics when custom labels are set

[CaptainAni187]

Fixes #2529

Problem

custom_labels_ is stored as a list ordered by topic. set_topic_labels converts a dict argument into a list before assigning it, so the attribute is never a mapping:

https://github.com/MaartenGr/BERTopic/blob/master/bertopic/_bertopic.py#L2028-L2040

delete_topics however iterated it as one:

for old_topic, label in self.custom_labels_.items()

so calling delete_topics() on a model with custom labels raised AttributeError: 'list' object has no attribute 'items'.

A second instance of the same assumption sits in the branch that initialises the -1 topic. self.custom_labels_[-1] = "" overwrites the last topic's label instead of adding one for the newly created outlier topic. That one is silent, and currently masked because the crash above always happens first — fixing only the reported error would have exposed it.

Fix

• Record the topics custom_labels_ is ordered by before the topic mapping is rebuilt, then remap the list positionally and re-emit it in new-topic order.
• Prepend the outlier label with insert(0, ""), since -1 sorts first, and keep the tracked topic list in step.
• Only remap when the label count matches the topic count, mirroring the existing guard in get_topic_info.

The rest of the surrounding code is unchanged — the other attributes really are dicts, so their comprehensions still apply.

Tests

Adds test_delete_with_custom_labels to tests/test_reduction/test_delete.py, parametrised over the same six fixtures as the existing test_delete. Each topic is labelled after itself, so misalignment is detectable: the test asserts that every surviving topic keeps its own label across renumbering, that a newly created -1 receives an empty label rather than stealing another topic's, and that the list length tracks the topic count.

• With the fix: 12 passed
• With the fix reverted (test kept): the 6 new cases fail

ruff check and ruff format are clean at the pinned v0.14.0.