Fix expert messages reactivity when switching between agent modes

[cstns]

Description

The message component was using array indices as key props. When switching between different agent modes, the underlying data changed but the indices remained the same (0,1,2...). This caused the framework to incorrectly reuse DOM elements, leading to rendering artifacts and stale state.

I updated the message rendering logic to use each message's unique uuid as the key. This ensures that the UI correctly disposes of old message components and mounts new ones when the agent context changes.

Related Issue(s)

introduced by #6839

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production
• Link to Changelog Entry PR, or note why one is not needed.

Labels

• Includes a DB migration? -> add the area:migration label

[dimitrieh]

Reported at https://flowfuse.slack.com/archives/C09QS57R8AD/p1773911852313069?thread_ts=1773865648.690409&cid=C09QS57R8AD

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.62%. Comparing base (f4ecf2f) to head (0d76d96).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6894   +/-   ##
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  Coverage   76.62%   76.62%           
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  Files         399      399           
  Lines       20165    20165           
  Branches     4856     4856           
=======================================
  Hits        15452    15452           
  Misses       4713     4713           

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[Steve-Mcl]

Makes sense but I cannot test in pre-staging so hanging back on merging.

However, I accept it is a small / somewhat inconsequential PR so please feel free to merge.