validate: flag a parameter whose type union mixes structured and scalar (#1055) (#1103)

Reports at WARN when a parameter's schema type union mixes a structured type
(array/object) with a scalar, whose style serialization is ambiguous
(e.g. type: [array, integer]: is ?token=5 the array ["5"] or the integer 5).
null is excluded (carries no serialization) and two scalars are fine. Uses
openapi3.WalkParameters to visit every parameter once (components, path items,
operations, callbacks, webhooks). ambiguous-parameter-serialization joins both
the registry and nativeRuleIDs (it is an oasdiff-native lint, not a kin code).

Fixes #1055