fix(openapi3): emit @encode decorator for anyOf/oneOf with unixtime format

[Copilot]

When importing OpenAPI schemas with anyOf/oneOf containing {type: "integer", format: "unixtime"} and {type: "null"}, the converter was dropping the @encode(DateTimeKnownEncoding.unixTimestamp, integer) decorator.

Before:

finished_at:
  anyOf:
    - type: integer
      format: unixtime
    - type: "null"

finished_at?: utcDateTime | null  // Missing @encode decorator

After:

@encode(DateTimeKnownEncoding.unixTimestamp, integer)
finished_at?: utcDateTime | null

Changes

• decorators.ts: Modified getDecoratorsForSchema to extract format property from anyOf/oneOf members when not present at schema root
• Tests: Added coverage for both anyOf and oneOf union variants with unixtime format
• Changelog: Added fix entry for @typespec/openapi3

Original prompt

---

This section details on the original issue you should resolve

<issue_title>converter - import anyOf unixtime missed decorator</issue_title>
<issue_description>When importing this OpenAPI schema

components:
  schemas:
    FineTuningJob:
      type: object
      properties:
        created_at:
          type: integer
          description: The Unix timestamp (in seconds) for when the fine-tuning job was created.
          format: unixtime

I get the following result

@encode(DateTimeKnownEncoding.unixTimestamp, integer) created_at: utcDateTime

However, if the schema is instead an anyOf with null, I get the following TypeSpec result, which misses the encode decorator

components:
  schemas:
    FineTuningJob:
      type: object
      properties:
        finished_at:
          anyOf:
            - type: integer
              description: The Unix timestamp (in seconds) for when the fine-tuning job was finished. The value will be null if the fine-tuning job is still running.
              format: unixtime
            - type: "null"

finished_at: utcDateTime | null

The decorator should be present whenever a union type with null is emitted.</issue_description>

<agent_instructions>run the formatting script before any commit.
when you're done with all the changes, run npx @chronus/chronus add to add a bugfix entry to the changelog.
use conventional commits.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes converter - import anyOf unixtime missed decorator #9163

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commit: 00205b9

[github-actions]

All changed packages have been documented.

• ✅ @typespec/openapi3

Show changes

@typespec/openapi3 - fix ✏️

Import OpenAPI schemas with anyOf/oneOf containing unixtime format correctly emits @encode(DateTimeKnownEncoding.unixTimestamp, integer) decorator for nullable utcDateTime properties