Bug: fix VP JWT marshalling flattening single-entry arrays incorrectly

[reinkrul]

We used JSON-LD marshalling semantics for JWT VPs, which is incorrect; single-entry arrays shouldn't be flattened to singular values.

Found during LSPxNuts hackaton.

This applied to the following fields:

• type
• verifiableCredential
• @context

Fixes #3957

[qltysh]

Coverage Impact

This PR will not change total coverage.

Modified Files with Diff Coverage (2)

Rating	File	% Diff	Uncovered Line #s
	vcr/holder/presenter.go	100.0%
	vcr/pe/presentation_submission.go	100.0%
	Total	100.0%

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