Fix expert chat input focus on drawer open and after MQTT response

[cstns]

Description

• Focus the chat textarea (not the drawer container) when the expert drawer is manually opened, so the user can start typing immediately
• Refocus the chat input after a response completes, using a watcher on isWaitingForResponse instead of chaining .then() on handleQuery(). The old approach relied on the HTTP promise lifecycle where handleQuery resolves after the full response. Over MQTT, sendMqttQuery returns immediately, so the .then() refocus fired before the response arrived and any DOM updates from the actual response would blur the input with nothing to restore it.

Related Issue(s)

closes #7931

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.62%. Comparing base (b327fda) to head (c6f9cf2).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7933   +/-   ##
=======================================
  Coverage   75.61%   75.62%           
=======================================
  Files         434      434           
  Lines       23232    23235    +3     
  Branches     6172     6174    +2     
=======================================
+ Hits        17568    17571    +3     
  Misses       5664     5664           

Flag	Coverage Δ
backend	75.62% <ø> (+<0.01%)	⬆️

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