fix(codegen): traverse references in all supported OpenAPI fields

Summary

Extend the codegen reference walker so component pruning discovers references
from all SchemaRef-bearing fields supported by the current kin-openapi model,
as well as from webhooks and nested media-type content.

Previously, a component referenced only from an OpenAPI 3.1 JSON Schema keyword,
an OpenAPI 3.2 itemSchema, a webhook, header content, or an encoding header
could be treated as unused and removed from components.

Problem

pruneUnusedComponents determines which components are reachable through
findComponentRefs, which in turn uses the shared walkSwagger traversal.
That traversal covered the original OpenAPI 3.0 schema locations but did not
keep pace with fields added to kin-openapi for OpenAPI 3.1 and 3.2.

For example, the reference in this OpenAPI 3.1 schema was not visited:

components:
  schemas:
    Text:
      type: string
    Container:
      type: object
      propertyNames:
        $ref: '#/components/schemas/Text'

The same problem affected references under prefixItems, contains,
patternProperties, dependentSchemas, unevaluatedItems,
unevaluatedProperties, if/then/else, $defs, and contentSchema.

Root-level webhooks were also skipped. In addition, media-type traversal did not
visit OpenAPI 3.2 itemSchema, schemas inside Header.Content, or headers nested
under MediaType.Encoding.

Changes

• Visit root-level webhooks using the same path-item traversal as normal paths.
• Continue walking components when an OpenAPI 3.1 document omits paths.
• Centralize traversal of child schema references in schemaChildRefs.
• Cover every SchemaRef-bearing field currently exposed by
  openapi3.Schema, including the OpenAPI 3.1 JSON Schema keywords.
• Centralize media-type traversal in walkContent.
• Visit both schema and OpenAPI 3.2 itemSchema in media types.
• Visit schemas and examples nested in parameter, request-body, response, and
  header content.
• Visit headers nested under media-type encodings.
• Reuse path-item traversal for callbacks.

Scope

This change does not add a local workaround for kin-openapi's lossy
SchemaRef.MarshalJSON behavior with OpenAPI 3.1 $ref sibling keywords.
Preserving the original sibling fields, including explicitly empty values and
self-referential schemas, requires a fix in kin-openapi because the relevant
source-presence information is stored in private SchemaRef fields.

Once kin-openapi preserves those fields during marshaling, oapi-codegen's
existing embedded-spec flow can continue to internalize external references and
marshal the document without clearing $ref or implementing its own cycle
detection.

This pull request was created with the assistance of OpenAI Codex.