openapi: format/enum/const/required/additionalProperties beside a $ref vanish

[OmarAlJarrah]

Summary

In the 2020-12 dialect keywords beside a $ref apply conjunctively. Constraint keywords beside a
$ref are kept (minimum, maximum, pattern, minLength, maxLength, minProperties …), and
so are the annotation keywords (description, title, deprecated, examples, default,
readOnly, x-*) and the validation-only ones (not). But five keywords that are neither reach
the IR in no form at all, with no Unmodeled entry and no diagnostic:

• format
• enum
• const
• required
• additionalProperties

The node lowers to a bare alias Scalar whose only content is Base pointing at the referent. The
declaration is gone.

Both positions behave the same way, so this is the alias lowering rather than one carrier: a
component whose body is $ref + siblings, and a property whose schema is $ref + siblings.

Reproduction

openapi: 3.1.0
info: {title: T, version: "1.0.0"}
paths: {}
components:
  schemas:
    BaseStr: {type: string}
    BaseObj: {type: object, properties: {a: {type: string}}}

    # Controls — the same keywords with no $ref beside them.
    PlainFmt:   {type: string, format: email}
    PlainEnum:  {type: string, enum: ["aa", "aaa"]}
    PlainConst: {type: string, const: "aa"}
    PlainReq:   {type: object, properties: {a: {type: string}}, required: [a], additionalProperties: false}

    # Beside a $ref.
    RefFmt:   {$ref: '#/components/schemas/BaseStr', format: email}
    RefEnum:  {$ref: '#/components/schemas/BaseStr', enum: ["aa", "aaa"]}
    RefConst: {$ref: '#/components/schemas/BaseStr', const: "aa"}
    RefShape: {$ref: '#/components/schemas/BaseObj', required: [a], additionalProperties: false}

Compiling this reports zero diagnostics. The controls lower as expected — PlainFmt gets
Encoding{name: email}, PlainEnum an Enum with two members, PlainConst a Literal,
PlainReq a closed Model with a required. Each Ref* alias gets Base and nothing else: no
encoding, no members, no literal, no additional, no constraints, no unmodeled.

The property position is identical — a property whose schema is
{$ref: '#/components/schemas/BaseStr', format: email, enum: [...], const: "aa"} keeps the
constraint siblings on the property and drops these five just as silently.

Why this is not one of the neighbouring issues

• compilers/openapi: $ref-adjacent siblings on an allOf branch are silently dropped #143 (closed) is the same class one position over: siblings on an allOf branch. Its fix
  did not reach the plain alias position, and its reasoning ("a $ref branch gets its own node")
  is about branches rather than about a component or property whose whole body is a $ref.
• openapi: first-match schema dispatch silently drops co-declared composition keywords #35 is the dispatch election between competing families (const → enum → allOf) when
  there is no $ref.
• openapi: keywords the elected lowering never reads are dropped without a trace #268 is keywords the elected lowering never reads, and explicitly notes that
  unhomedKeywords "only flags format when no type is declared beside it". In the alias case no
  type is declared, so that guard should let format through, and it does not.
• openapi: numeric exclusiveMinimum/exclusiveMaximum overwrites a tighter co-declared minimum/maximum #33 is the numeric-bound clobber, a different mechanism entirely (a keyword that is read,
  overwritten by another).

Expected

Per invariant 2, a keyword the alias lowering does not consume is kept verbatim under Unmodeled
with a diagnostic naming it, the way the constraint and annotation siblings at the same position
already are. Silence is what makes this one costly: an enum or a const beside a $ref is a
narrowing idiom, so the generated SDK admits values the source forbids and nothing says so.

Found while reviewing #272, which covers a different set of drops in the operation walk and does
not touch the schema alias lowering.