fix: clean shared messages state in case of history rewrite

[asvishnyakov]

Because the messages field is cumulative, overwriting history in any of the swarm agents causes the history update (such as a message summary) to be appended to the end. For example:

Agent 1:

• Message 1
• Message 2
• Hand off to Agent 2

Agent 2:

• Message 1
• Message 2
• Hand off to Agent 2
• Message 3

Then Agent 2 overwrites the history:

• Summary
• Message 4

And when handing off back to Agent 1, this happens:

• Message 1
• Message 2
• Hand off to Agent 2
• Summary
• Message 4
• Hand off to Agent 1

This not only disrupts the flow of the conversation but also causes issues if the summary is a system message (such as in langmem's SummarizationNode).

On the other hand, if history is not overwritten, everything continues to work correctly after changes made in this PR, since the "removed" messages will simply be replaced by themselves.

Fixes #84.

[asvishnyakov]

@farisnanosoft @sydney-runkle @eyurtsev I’d appreciate any feedback if you think this isn’t the right fix for the issue, as I’m currently experiencing it in my project.

[asvishnyakov]

@sydney-runkle May you look into this please?

[asvishnyakov]

@sydney-runkle @eyurtsev May someone take a look on this, please?