[7124] Add response-validation & surface response schema drift in dev

[n-lark]

Description

This is a subtask of #7083.

This PR is scoped to establishing a baseline — it adds the package, wires it up in dev mode, and fixes the drift the new validator surfaces. It does not add required arrays or additionalProperties: false to any schema; that work lands in a follow-up PR on top of this one.

Related Issue(s)

Resolves #7124

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 76.34%. Comparing base (5d011ef) to head (3b49293).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7125      +/-   ##
==========================================
+ Coverage   76.29%   76.34%   +0.04%     
==========================================
  Files         405      405              
  Lines       20348    20390      +42     
  Branches     4895     4918      +23     
==========================================
+ Hits        15525    15567      +42     
  Misses       4823     4823              

Flag	Coverage Δ
backend	76.34% <100.00%> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[cstns]

looking good overall, just a few questions here and there