feat: extend DBFlexContextSchema validation to only allow valid units

[joshuaunity]

Description

This PR extends the FlexContextSchema validation to also cover for field value unit validations

Look & Feel

No visual description as it's a backend

How to test

1. Go to asset's page and click on an asset
2. Go ahead to edit the asset's flex-context
3. Try any of the following

• Add a non-energy price unit to any price field
• Add a non-power unit to any power field

Further Improvements

In this PR I noticed a rarely occurring bug with the modal UI, a follow up PR will be made to fix this bug and another one reported by @nhoening

Related Items

This PR closes #1353

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• I agree to contribute to the project under Apache 2 License.
• To the best of my knowledge, the proposed patch is not based on code under GPL or other license that is incompatible with FlexMeasures

[joshuaunity]

@nhoening @Flix6x

I followed the discussion on the issue ticket, but at the en,d it wasn't clear where we decided to put this validation, the FlexContextSchema or the DBFlexContextSchema

[Flix6x]

DBFlexContextSchema

[nhoening]

Thanks, this works well, I have a comment on making the code more explicit.

[nhoening]

Thanks, I'm happy!

(aside from one small docstring clarification about future work)

[Flix6x]

Thank you for adding a lot of test cases! This made it easier to spot that we are not yet distinguishing correctly between energy prices and capacity prices.

[Flix6x]

Thanks, the new function works well. Thanks for adding the doctests, too. I only have a few more requests mostly regarding serializing the field names in error messages of the price fields.

[Flix6x]

Job well done! Again, the addition of the various test cases really helped us to prevent introducing bugs, thank you for adding those. Units are tricky, but really well suited for unit tests (what's in a name).

Please note that I removed a validator that I suspected to be redundant. Specifically, passing a time series specification (as a list of dicts) to the DBFlexContextSchema already lead to a ValidationError, as you can see here, where the same test case (also) triggered an alternative message. I actually touched up that alternative message in e48a763 and 6065a5a.

I also made several corrections to distinguish energy prices from capacity prices.