chore: Part 1 of adding type validation to tests - type checking test/core/pipeline/ folder

[sjrl]

Related Issues

• part of Run typing checks on our test files #10396

Proposed Changes:

Add type checking to the test/core/pipeline/ folder.

How did you test it?

Running typing tests on the newly added folder.

Notes for the reviewer

Checklist

• I have read the contributors guidelines and the code of conduct.
• I have updated the related issue with new insights and changes.
• I have 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: and added ! in case the PR includes breaking changes.
• I have documented my code.
• I have added a release note file, following the contributors guidelines.
• I have run pre-commit hooks and fixed any issue.

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1 Skipped Deployment

Project	Deployment	Review	Updated (UTC)
haystack-docs	Ignored		Jan 20, 2026 3:16pm

[sjrl]

Only change needed outside of test folder. Without this then passing something like type=int | str would cause mypy to throw an error.

[anakin87]

I see there's a type error.

(Over time, let's also check if mypy running time increases significantly)

[sjrl]

I see there's a type error.

(Over time, let's also check if mypy running time increases significantly)

@anakin87 I'll need to investigate this more about how we can properly support this. I'm not too familiar with how python treats type but it looks like UnionType doesn't count as a type which cascades down into more issues in our type_util methods. I'll see if there is a reasonable way to get it working short of setting type: Any