Support discriminator for oneOf in OpenApiValidationPlugin #2040

OpenApiValidationPlugin (#2037) validates oneOf strictly: a body that matches more than one branch is reported. That's correct per spec, but many real-world specs have overlapping oneOf branches and rely on discriminator to pick one, so valid payloads get flagged.

Proposal: when a discriminator is present, select the branch from the discriminator property's value, using mapping (or the implicit schema name if there's no mapping), and validate only that branch. Report a clear error when the discriminator value is missing or unknown.

Depends on #2037.