chore: Optimum - use new import path for SentenceTransformerPoolingLayer

[anakin87]

Related Issues

• failing mypy checks: https://github.com/deepset-ai/haystack-core-integrations/actions/runs/24320307912/job/71034106888

The problem is that Sentence Transformers now has Pooling in a different path. They keep backward compatibility but this does not play well with mypy.

Proposed Changes:

• use the new import path if available
• fall back to the old import path to keep compatibility with older Sentence Transformers version

How did you test it?

CI

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test:.

[github-actions]

Coverage report (optimum)

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
integrations/optimum/src/haystack_integrations/components/embedders/optimum
_backend.py					34-35
Project Total

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