fix: correct representative docs sampling and index mapping in _extract_representative_docs

[pidefrem]

What does this PR do?

Fixes two related correctness bugs in BERTopic._extract_representative_docs(). They are independent but live in the same method, so they're addressed together.

Bug 1 — duplicate representative documents from replace=True sampling

_extract_representative_docs sampled with sample(n=nr_samples, replace=True) followed by .drop_duplicates(). When a topic has fewer unique documents than nr_samples, replace=True draws the same document repeatedly; the trailing .drop_duplicates() runs after sampling and doesn't prevent it. The duplicates inflate the c-TF-IDF similarity calculation and can leave the same document appearing multiple times in representative_docs_.

Fix: de-duplicate per (Topic, Document) before the groupby, and sample without replacement capped at the group size — sample(n=min(nr_samples, len(group)), replace=False).

Bug 2 — text-based in matching maps documents to the wrong topic

Selected documents were mapped back to their original indices with a text membership test:

doc_ids = [selected_docs_ids[index] for index, doc in enumerate(selected_docs) if doc in docs]

When the same text appears in more than one topic (short texts, boilerplate, near-duplicates), doc in docs matches the first occurrence regardless of topic — producing skipped documents, misaligned doc_ids ↔ selected_docs pairs, and representative_docs_ containing documents from the wrong topic.

Fix: track the positional indices returned by the similarity/MMR selection and use them to look up doc_ids, instead of text membership. Both the MMR and non-MMR branches are corrected.

Testing

Added two test modules (11 tests, all passing):

• tests/test_dedup_representative_docs.py — no duplicate representative docs per topic, including the MMR branch and topics smaller than nr_samples.
• tests/test_repr_docs_indexing.py — representative docs map to the correct topic when identical text spans multiple topics.

Defaults are unchanged and the change is backward compatible.

Fixes #2495

Before submitting

• This PR fixes a typo or improves the docs (if yes, ignore all other checks!).
• Did you read the contributor guideline?
• Was this discussed/approved via a Github issue? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes (if applicable)?
• Did you write any new necessary tests?