openapi: first-match schema dispatch silently drops co-declared composition keywords

[OmarAlJarrah]

Summary

The schema lowering dispatch picks one keyword and never revisits the rest of the schema body, silently dropping legal JSON Schema conjunctions in two shapes:

1. oneOf + anyOf co-declared. buildUnion (and nullUnionCollapse) picks oneOf when non-empty and only falls back to anyOf when oneOf is empty. hasUnionSiblings does not treat the co-present other union keyword as a sibling, so the losing branch set gets no Extensions entry and no diagnostic.
2. allOf co-declared with enum/const. lower() dispatches on const, then enum, then allOf — first match wins. The narrowing idiom allOf: [$ref: Base] + enum: [...] loses the composition entirely: no Base/Mixin, no preservation, no diagnostic.

Both violate invariant 2 (lossless by default) — contrast with the oneOf/anyOf-next-to-structural-keywords case, which lowerWithUnionSiblings carefully preserves.

Reproduction

# case 1 — anyOf vanishes
S: {oneOf: [{type: string}, {type: integer}], anyOf: [{type: number}, {type: boolean}]}
# → Union with 2 variants, Extensions empty, zero diagnostics

# case 2 — allOf vanishes
Constrained: {allOf: [{$ref: '#/components/schemas/Base'}], enum: [a, b]}
# → plain Enum{a,b}; relationship to Base gone, zero diagnostics

Root cause

compilers/openapi/compose.go:178-182 (buildUnion preference), schema.go:152-169 (hasUnionSiblings blind to the other union keyword), schema.go:300-319 (const→enum→allOf precedence).

Expected

When a schema co-declares keywords the dispatch cannot jointly model, the losing keyword set must be preserved raw under Extensions with a degraded-construct diagnostic — the same treatment lowerWithUnionSiblings already gives structural siblings. Silence is never acceptable here.