json parser: collapse the parser indirection into a single JsonParser implementation #634

What

JsonParser::FromFile and FromBuffer are not defined alongside the class they belong to. Each parser implementation supplies its own definitions of those symbols, and exactly one gets linked. That is a link-time substitution point whose only purpose was letting a second implementation take over the same symbols.

After #632 is done and only one parser is left, the indirection has no reason to exist and the extra parser class is a second name for something that already has one. Make JsonParser own its implementation directly, and reduce the vaJson Any tree builder to an internal detail with no separate identity in the score::json namespace.

Public behavior and API must not change. This is a structural change only.

Acceptance Criteria (DoD)

JsonParser's methods are defined together with the class they belong to

How

No response