fix: keep the referenced type when a $ref has complex siblings - #1832

A schema that carries a $ref next to not, allOf, oneOf or anyOf is routed to ComplexSchemaParser, which parses the complex keyword and drops the reference. With not — the one complex keyword that has no TypeScript equivalent, so NotSchemaParser returns any — the whole property collapses:

constrainedRef:
  $ref: '#/components/schemas/Status'
  not: { enum: [STATUS_UNSPECIFIED] }

// before
constrainedRef?: any;
// after
constrainedRef?: Status;

Sibling keywords beside a $ref are legal in OpenAPI 3.1, where they compose with the referenced schema. They are also what protoc-gen-connect-openapi emits for a protobuf enum field carrying a protovalidate not_in rule — the common "must not be the UNSPECIFIED member" constraint. In a 678-operation document generated that way, this turned 156 properties into any; with this change the same document generates 11, all of them genuinely dynamic (the client's own contentFormatters, a map value, google.protobuf.Any).

The fix parses the reference alongside the complex content and filters any out of the intersection, reusing the ignoreTypes idiom AllOfSchemaParser already applies. A $ref with only annotation siblings (title, description) took the primitive path and already resolved correctly; that behaviour is unchanged, and the new fixture covers all three shapes.

Tests: tests/spec/ref-with-sibling-keywords added. vitest run goes from 283 to 284 passing with no snapshot churn elsewhere; the 5 failures in paths-2 and paths-2-prefer-existing-schema-names fail identically on a clean checkout of main.

Summary by cubic

Fixes $ref schemas with complex siblings like not, allOf, oneOf, or anyOf so the referenced type is preserved instead of collapsing to any.

• Sibling keywords are legal in OpenAPI 3.1 and are what protoc-gen-connect-openapi emits for protobuf enum fields with not_in rules.
• The reference is now parsed alongside the complex content, and any members are filtered out of the intersection.
• In a 678-operation generated document, this restores types on 156 properties; only genuinely dynamic fields remain any.
• A $ref with only annotation siblings already resolved correctly and is unchanged.
• Adds a test fixture covering plain refs, annotated refs, and refs with not.

^{Written for commit c3a86d4. Summary will update on new commits.}