fix: prevent in-place mutation of message history in normalizer

[pipe1os]

Description

Fixes a state corruption bug where _normalize_messages mutated the agent.messages history in place during streaming. This permanently altered the canonical conversation state before it reached the session manager. Changed the normalization loop to build shallow copies so the original message history remains pristine.

Related Issues

N/A

Documentation PR

N/A

Type of Change

Bug fix

Testing

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly - N/A, internal bug fix
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed - N/A, internal bug fix
• My changes do not introduce any new warnings

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