Make CVSS4.as_json return valid json and make "minimal" arg work.

[jsvob]

Closes issue #74
JSON Schema: https://www.first.org/cvss/cvss-v4.0.json

[jsvob]

The official json schema doesn't check for missing mandatory metrics nor for extraneous invalid metrics. A workaround to manually check the validated json against the schema is to make one value invalid and see that the schema doesn't validate (= it's a metric known to the schema), and do this separately for each metric, thus manually validating that each generated dictionary key / cvss v4 json metric has a valid name.

[tschmidtb51]

The official json schema doesn't check for missing mandatory metrics nor for extraneous invalid metrics. [...]

Maybe, I'm missing something but wouldn't it be easier to either:

• use a modified version of the schema (converted to Draft 2020-12) and set the "unevaluatedProperties": false (see https://github.com/oasis-tcs/csaf/blob/editor-revision-2025-05-28/csaf_2.1/referenced_schema/first/cvss-v4.0_strict.json)
• compute the Score and Severity from the vector itself and test whether the value is the same?

The latter one has the benefit that one can be sure that the JSON is also semantically valid.