openapi3: record scalar-valued map keys for origin location lookup

[reuvenharrison]

What

Record the per-key locations of a scalar-valued map (e.g. an OAuth flow's scopes) on the parent struct's Origin, so a consumer can locate an individual entry by key.

Why

A scalar-valued map such as

scopes:
  read:pets: read your pets
  write:pets: modify pets in your account

decodes into a Go map[string]string, which has no Origin field of its own. The per-key locations are present in the OriginTree, but applyOriginsToMap discards them for such maps (the values are plain strings with nowhere to hang an origin). As a result there was no way to point at an individual scope.

How

In applyOriginsToStruct, when a struct field is a scalar-valued map, copy its keys' locations from the child subtree onto the struct's own Origin.Sequences[field] (the same shape already used for sequence items), addressable by key name. Consumers read them via the existing sequence-item lookup.

Gated by isScalarValuedMapField, so it fires only for maps whose element is a scalar (string/bool/number) — scopes, discriminator.mapping, x- string maps. Pointer- or struct-valued maps (e.g. properties: map[string]*SchemaRef) are untouched, because their values already carry their own Origin via the normal recursion. No yaml3 change is needed: the key locations are already in the OriginTree.

Tests

TestOrigin_Security now asserts Implicit.Origin.Sequences["scopes"] resolves read:pets/write:pets to their exact lines. Full openapi3 suite passes.