Enforce cache key is a string

[Steve-Mcl]

Description

What does this PR do?

Makes the app's cache system reject non-string keys instead of silently misbehaving.

Why was this needed?

The app can store its short-lived cache data in one of two ways: an in-memory cache (used locally and in some environments) or a Redis/Valkey cache (used in production). These two behave differently if you accidentally use something other than a plain string as a cache key, e.g. a number instead of a string. The in-memory cache will quietly accept it and appear to work fine, but Redis handles keys differently and can fail in confusing ways.

This mismatch already caused a hard-to-track-down bug: something worked fine locally but broke in an environment using Redis, and it took a long time to figure out why, because the local setup didn't use Redis and couldn't have caught it.

What changed?

Both cache drivers now check that a key is a string before using it, and immediately throw a clear, descriptive error if it isn't. This means:

• The same mistake now fails the same way everywhere, whether running locally or in production.
• Problems like this get caught early, during development or testing, instead of surfacing later as a confusing production issue.

Testing

Added tests to confirm both cache drivers reject non-string keys consistently.

Related Issue(s)

closes #7666

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 76.14%. Comparing base (c2ae1d0) to head (65a222a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8019      +/-   ##
==========================================
+ Coverage   76.13%   76.14%   +0.01%     
==========================================
  Files         439      440       +1     
  Lines       23548    23560      +12     
  Branches     6272     6273       +1     
==========================================
+ Hits        17928    17940      +12     
  Misses       5620     5620              

Flag	Coverage Δ
backend	76.14% <100.00%> (+0.01%)	⬆️

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