Unique Particle Attribution checked only a complexContent extension's own
particles, missing the cross-boundary non-determinism with its base: the
effective content of an extension is the base type's content followed by the
extension's, so a base particle that can recur or end optionally competes
with the extension's leading particles. An element matching either was a UPA
violation the single-node check did not see, and the schema was wrongly
accepted (XSTS particlesZ022).
Build the content model over the effective sequence (base content prepended,
resolved recursively). The base is followed only for a local
(target-namespace), non-cyclic base chain; a foreign, built-in (xs:anyType),
unresolved, or cyclic base leaves the check on the extension's own content, so
no model that cannot be fully assembled is rejected (an under-rejection, never
a false positive). For a non-extension the effective list is a single node, so
sequence([x]) reproduces the prior automaton exactly.
Verified against the full W3C XSTS 2006-11-06 suite: invalid-schemas-accepted
31 -> 29 (particlesZ022 and one more), valid-schemas-rejected held at 0, no
other bucket moved. Adds UPA tests for an ambiguous extension and a
deterministic one (the over-rejection guard); moves the content-model-node and
effective-model helpers to a new file to stay within the type-body and
file-length budgets.
