fix: validate modules during callable traversal

[sjrl]

Related Issues

• fixes #issue-number

Proposed Changes:

Harden callable deserialization by checking the real module of every object traversed in a dotted callable path. This prevents an allowlisted module from exposing an object defined in an unallowlisted module that leads back to an otherwise allowlisted final callable.

How did you test it?

New test

Notes for the reviewer

@julian-risch this should stop the POC you developed by accessing os.environ through a module loaded in HITL

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	Actions	Updated (UTC)
haystack-docs	Ignored	Preview	Aug 20, 2026 8:59am

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Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
haystack/utils
callable_serialization.py					117-119
Project Total

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