[forms] Validate value length for value maps

[elpaso]

Display errors and trim the values.

This also works when importing from CSV or from layer.

Fix #57634

[github-actions]

🪟 Windows builds ready!

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(Built from commit d4cf6b0)

[roya0045]

This widget always had an issue with unconstrained inputs. I agree with the proposed approach, though the more arduous problems are data mismatch. The widget will accept any value, even if the field type doesn't support this value.

I don't know if this is good to include in this pr or as another one.

[elpaso]

This widget always had an issue with unconstrained inputs. I agree with the proposed approach, though the more arduous problems are data mismatch. The widget will accept any value, even if the field type doesn't support this value.

I don't know if this is good to include in this pr or as another one.

I think this PR is a step forward, in the sense that it provides the API to call validation and to display errors to the user.

Given the limited amount of time that I have left for this bugfixing round I'd rather prefer to keep the scope limited to fixing this particular issue, more validation rules could be added later.

It would be nice to have a new ticket that lists the missing validation rules, for example the type compatibility, and I'm not sure if there is a check for unique values and descriptions.