fix(pipeline): preserve dictionary subclasses when copying inputs

[hahahahahayesyeseys]

Related Issues

• fixes _deepcopy_with_exceptions turns dict subclasses (defaultdict, OrderedDict, Counter) into plain dict, so pipeline inputs lose their type #13092

Proposed Changes:

_deepcopy_with_exceptions now preserves dictionary subclasses when copying pipeline inputs. Plain dictionaries retain their current path; subclasses are copied while retaining their type and state, then recursively populated with copied values. This keeps behaviors such as defaultdict.default_factory, OrderedDict ordering, and Counter semantics.

A regression test covers OrderedDict, defaultdict, and Counter, including missing-key creation for defaultdict. A release note documents the user-visible fix.

How did you test it?

• hatch run test:unit test/core/pipeline/test_utils.py -q --tb=short (28 passed)
• hatch run test:unit test/core/pipeline -q --tb=short (442 passed, 155 deselected)
• hatch run test:types haystack/core/pipeline/utils.py test/core/pipeline/test_utils.py
• hatch run fmt-check haystack/core/pipeline/utils.py test/core/pipeline/test_utils.py
• hatch run pre-commit run --files haystack/core/pipeline/utils.py test/core/pipeline/test_utils.py releasenotes/notes/preserve-dict-subclasses-67205ab6180303fa.yaml
• git diff --check

Notes for the reviewer

The issue is open, unassigned, and has no linked branch or pull request at the review gate. The patch follows the issue's proposed copy/clear/repopulate approach and leaves the existing plain-dict fallback available for custom subclasses that reject copying. The full repository suite was not run.

This PR was fully generated with an AI assistant. I have reviewed the changes and run the relevant tests.

Checklist

• I have read contributors guidelines/code of conduct.
• I have updated the related issue with the proposed changes.
• I have added unit tests and updated the release note.
• The PR title uses the conventional commit format.
• The changed code is documented by the release note and existing API documentation remains accurate.
• Pre-commit hooks pass.

[vercel]

@hahahahahayesyeseys is attempting to deploy a commit to the deepset Team on Vercel.

A member of the Team first needs to authorize it.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[github-actions]

Hi @hahahahahayesyeseys, thanks for your interest in contributing to Haystack! 🙏

⚠️ Issue #13092 is already being addressed by open pull request(s) #13094. Before opening a PR for an issue, please check whether a PR is already linked to it, and consider contributing to the existing PR instead. We may close duplicate PRs to keep the review queue manageable.

This is an automated message to help us keep the review queue healthy.

[inchang-ing]

Heads-up so we don't land the same fix twice: I have #13094 open against the same problem (it touches haystack/core/pipeline/utils.py with the same +11/-2 change: copy(obj) + refill to keep the concrete dict subclass, plain dict keeps the current comprehension path). Mine predates this one, is CLA-signed and CI-green.

Whichever of the two you merge, feel free to close the other — I'm happy for #13096 to supersede mine if you prefer it. Just leave a note and I'll close mine myself rather than leaving a duplicate open.