Refactor creating `parameter_value` as JSON

[suvayu]

Currently creating the parameter_value as JSON essentially "reimplements" by crafting the dictionaries by hand. Refactor it to use the Pydantic dataclasses, see: spinedb_api/parameter_value.py::to_list.

Existing tests should already cover this code, so a refactor should not break any tests.

Error handling

Parse pydantic errors to something friendlier, and raise ParameterValueFormatError.

[soininen]

We actually have Codecov tracking test coverage; looks like spinedb_api.parameter_value.to_list() is mostly covered except for the JSONDecodeError catch block around line 2275. You can find links to the coverity results in the PR.

On a side note, the test coverage for the WIP-data-transition patch is 86% --- we should aim for higher.