feat: [2271] Dashboard data store - Test harness for reading from the store

[n-lark]

Description

First of three PRs that move reads onto the data store (#2239). The store has only ever received copies of what the legacy datastore works out, so there's no way yet to prove it behaves the same. This adds the test harness that will.

A copy of today's legacy datastore acts as the reference, and sequences of datastore calls modeled on every core widget are run through both and compared after every step. After each step the harness also mutates what it passed in and what get() returned, then compares again, so a datastore that keeps or hands out its own objects fails. This PR proves the harness catches breakage; the next one uses it to check the store.

The only production change moves hasExtraProps from ui_base.js into nodes/utils so the harness uses the real one rather than a copy. Behaviour is unchanged.

Test plan

• data.js matches the reference after every step of all 315 sequences, including after the input and the returned value are mutated
• Deliberately breaking data.js fails the harness: merge, ui_update stripping, point limit, filter, clear, has(), client scoping, and cloning on save, append or get
• A harness that runs no steps fails

Related Issue(s)

Resolves #2271

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

[cstns]

Nice harness. I broke data.js a few ways and it catches merge, ui_update, point limit, filter, clear and has() changes easily.

One gap worth closing before the next PR leans on it: aliasing bugs pass untouched. Making get() return the live stored object, or saving an array without cloning, still gives 315 passing, since nothing mutates an object after handing it over or reading it back. Reading from the proxy store is exactly where that kind of bug would show up (onChange in ui_base mutates what datastore.get() returns), so a step that mutates the returned object and compares again might be worth adding.

Optional: client scoped saving is only covered by the one handwritten scenario (removing that check fails 1 of 315), so adding a scoped widget to generate() could help. And hasExtraProps is copied from ui_base rather than required, so it could drift.

[n-lark]

@cstns thanks - made these changes:

1. Each step now mutates the input and the value get() returns, then compares again. A live get() or a save or append that doesn't clone now fails.
2. Added scoped widgets to generate(). Removing the client-scoped check now fails 201 runs instead of 1.
3. Moved hasExtraProps to nodes/utils so the harness uses the real one.