feat(schema): revisit VAST 4.4 support for AdCP 3.3

[bokelley]

Context

PR #7230 proposed adding 4.4 to the VAST version enum. We are deferring that change from AdCP 3.2 because VAST 4.4 is not yet a published IAB Tech Lab specification:

• The VAST 4.4 document still says Publication Date TBD.
• vast_4.4.xsd identifies itself as a draft for working-group discussion.
• The finalized CTV Ad Portfolio signaling guidance uses version="4.2" in its concrete examples.
• The draft XSD has an unresolved content-model/cardinality issue: VAST 4.4: xs:choice minOccurs=0 maxOccurs=unbounded on vastInLine_type / vastWrapper_type drops all child cardinality InteractiveAdvertisingBureau/VAST#58.

There is early test activity, including VAST 4.4 fixtures and validator demos, but we have not found independent adopter evidence that AdCP currently blocks a real integration.

Revisit for 3.3

Add VAST 4.4 when either:

1. IAB Tech Lab publishes the final VAST 4.4 specification/schema, or
2. An independent adopter needs truthful version="4.4" declarations for a concrete interoperability test and can describe the compatibility expectations.

When revisiting, start from #7230 and revalidate the enum, tracker event/version gates, documentation examples, and changeset against the then-current IAB artifacts.

[aleksUIX]

Follow-up on the IAB side of this: InteractiveAdvertisingBureau/vast#59 now has in-tree xmllint fixtures for the three #58 cases (empty InLine, empty Wrapper, repeated AdSystem). Still waiting on IAB to land the schema fix.