fix(types): include optional metadata on ChatCompletion responses (#1818)

[Adityakk9031]

• I understand that this repository is auto-generated and my pull request may not be merged

Changes being requested

This PR addresses #1818 (Chat Completions is not returning the metadata).

Root cause

metadata is accepted in chat.completions.create params and supported in stored completion flows (retrieve, update, list), but the ChatCompletion response type did not include a metadata field.
This caused a contract/type mismatch in the SDK even when the API returns metadata for stored objects.

What changed

• Added metadata?: Shared.Metadata | null to ChatCompletion in:
  • src/resources/chat/completions/completions.ts
• Added contextual docs indicating metadata is commonly present on stored completions and may be absent in immediate create responses.

Scope

• Type/docs-only change.
• No runtime request/response parsing logic changed.