fix: docling stop relying on haystack.utils.base_serialization helpers

[davidsbatista]

Related Issues

• related to chore!: removing deadcode haystack#11905

Proposed Changes:

• Removes DoclingConverter's dependency on serialize_class_instance/deserialize_class_instance (being deleted from haystack-ai as dead code).
• Now uses component_to_dict/deserialize_chatgenerator_inplace for the meta_extractor field, and BaseMetaExtractor.to_dict/from_dict use default_to_dict/default_from_dict.
• Bumped minimum haystack-ai to 2.12.0 since deserialize_chatgenerator_inplace isn't available before that.

How did you test it?

• Updated test_converter.py fixtures; unit tests, ruff, mypy all pass, including lowest-dependency 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 (docling)

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
integrations/docling/src/haystack_integrations/components/converters/docling
converter.py
Project Total

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