Persistence extensions: preserve item unit in results

[mherwege]

See discussion: https://community.openhab.org/t/javascript-persistance-totals-off/165099

Many of the persistence extensions are internally converting to the system unit to make them indepent of unit offsets. The Quantity results returned where in this system unit, and not in the item unit. Typically, persisted values are stored in the item unit.

This PR tries to return the item unit (or a unit derived from it) as much as possible.

• sumSince, sumUntil and sumBetween will now always return the result in the item unit.
• riemannSumSince, riemannSumUntil riemannSumBetween will return the result in item unit * s.
• sumSince, sumUntil and sumBetween will now always return the result in the item unit.
• varianceSince, varianceUntil and varianceBetween return the result in item unit ^2.
• deviationSince, deviationUntil and deviationBetween return the result in item unit.

Depending on the item unit, the resulting unit may get simplified automatically. This does not happen for pre-defined composed units (such as kWh/s will not automatically eliminate the time dimension, but kW * h / s will).

The changes in this PR are changing the results of the riemannSum, variance, and deviation extensions for Temperatures as they will now be calculated using the item unit, and not the system unit for temperatures (K). I thought long about this, but I believe most people will expect these calculations not to be done in K. Note that evolutionRate was already doing the calculation in the item unit, making the persistence extension inconsistent.

[openhab-bot]

This pull request has been mentioned on openHAB Community. There might be relevant details there:

https://community.openhab.org/t/javascript-persistance-totals-off/165099/19

[Copilot]

Pull Request Overview

This PR updates persistence extensions to preserve the item unit in calculation results instead of converting to system units. Previously, methods like riemannSum, variance, and deviation were converting quantities to system units (e.g., Kelvin for temperatures) before calculations, which resulted in outputs in system units rather than the original item units.

Key changes include:

• Modified variance and deviation calculations to use item units instead of system units
• Updated Riemann sum calculations to preserve item units in results
• Removed Kelvin offset handling from test utilities
• Added comprehensive test coverage for EnergyPrice quantities

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
TestPersistenceService.java	Removes Kelvin offset handling and simplifies test value generation
PersistenceExtensionsTest.java	Updates test expectations to verify item units are preserved and adds EnergyPrice test coverage
PersistenceExtensions.java	Modifies core calculation logic to use item units instead of system units and adds documentation notes

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[openhab-bot]

This pull request has been mentioned on openHAB Community. There might be relevant details there:

https://community.openhab.org/t/persistence-in-rule/166016/3

[kaikreuzer]

@wborn Did you review this PR already?
How risky would you consider it to merge it for 5.1?

[wborn]

Haven't looked at the code yet @kaikreuzer. I only requested some initial @copilot feedback.

[mherwege]

Rebased