fix: require messages input parameter in Agent component

[sjrl]

Related Issues

• fixes Spurious component execution via pipeline_run_args injection #11109

Proposed Changes:

Make the messages param of Agent required.

Same issue as outlined here #10734

making Agent's messages input optional created unwarranted pipeline scheduling behavior

How did you test it?

Updated unit tests

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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Project	Deployment	Actions	Updated (UTC)
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Coverage Report for CI Build 24655141668

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Coverage: 92.855%

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Coverage Stats

Relevant Lines:	17173
Covered Lines:	15946
Line Coverage:	92.86%
Coverage Strength:	0.93 hits per line

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[julian-risch]

Code changes look good to me! I suggest we extend the release note with an upgrade section that tells users to add messages=[] if they didn't have messages for the Agent at all in Haystack 2.27 or Haystack 2.26. We can mention that only users of these two versions are affected.

[julian-risch]

I suggest we also include a note in the section upgrade given that this is a breaking change for users you started to use Agent without messages in the mean time. I don't expect many users to be affected if any at all.