feat!: Update ChatMessage to use to_dict and from_dict in pydantic serialization

[sjrl]

Related Issues

• fixes #issue-number

Proposed Changes:

How did you test it?

New tests

Notes for the reviewer

cc @tstadel

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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Project	Deployment	Actions	Updated (UTC)
haystack-docs	Ready	Preview, Comment	Jul 16, 2026 8:21am

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Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
haystack/dataclasses
chat_message.py
Project Total

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