Upgrade to Docling 2.9.0 or newer for HybridChunker

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We need to update our Docling dependency to 2.9.0 or newer as a prerequisite to using the HybridChunker. Preference on exact version should be the newest we can pick up at the time that aligns with what the instructlab/instructlab repo is using for Docling.

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Right now instructlab/instructlab is using docling-core[chunking]>=2.10.0 so we should ensure we align on versions with that repository. And, if we need to bump the minimum version (to get for example a fix for #514) then we should coordinate that with the core repo.

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Note that docling 2.12.0 (which adds GPU acceleration) pulls in different PDF models than previous versions used. This may have implication for any downstream users if we bump to that version or newer, so something we'll have to coordinate if we need to move to 2.12.0 or newer.

[aakankshaduggal]

The docling team has addressed two of our issues DS4SD/docling#822 with https://github.com/DS4SD/docling/releases/tag/v2.17.0 and DS4SD/docling#734 with https://github.com/DS4SD/docling/releases/tag/v2.18.0
We should aim to use v2.18.0 or above because the issues with HTML in markdowns was a pain point for some users.

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That seems reasonable, with the caveat that we should let our downstream users know as some of them will need to pull in those newer docling models in their distributions after we update.