Issue #962 - Added Test for C12

[kundyzs]

Description

This PR adds C12, ensuring that kWh readings are correctly converted to kg of CO₂ (kg CO₂). The test validates:

1-day bars for 15 minute reading intervals.
Correct application of the 0.709 conversion factor (kWh → kg CO₂).
Support for relevant unit conversions (u2, u10, u12) and chained transformations (c11, c12).
This update improves OED’s kg CO₂ calculations and ensures consistency with Electric_Utility-based unit conversions.

Contributors:
Kundyz Serzhankyzy - kundyzs
Kevin Jijon - kevJ711
Ye Chan Lin - yechanlin

Partly Addresses #962

Type of change

(Check the ones that apply by placing an "x" instead of the space in the [ ] so it becomes [x])

• Note merging this changes the database configuration.
• This change requires a documentation update

Checklist

(Note what you have done by placing an "x" instead of the space in the [ ] so it becomes [x]. It is hoped you do all of them.)

• I have followed the OED pull request ideas
• I have removed text in ( ) from the issue request
• You acknowledge that every person contributing to this work has signed the OED Contributing License Agreement and each author is listed in the Description section.

Limitations

No known limitations.

[Chocopepero]

Thanks to @kundyzs, @kevJ711, & @yechanlin for working on this issue. Everything looks good. Just need a final look from @huss and it's good to merge.

[huss]

Thanks to @kundyzs, @kevJ711 & @yechanlin for their first contribution to OED. The review by @Chocopepero was fine and taken for approval. Everything looks good. Congratulations on your first accepted work for OED.