feat(api): make function call output call IDs optional (#3738)

## Summary

Align function call output types with the API schema: `call_id` is
optional in stable and beta input and response types, and input types
also accept `null`.

Agents SDK change is expected / will need to be fixed after this PR
merges.

Co-authored-by: apcha-oai <228803254+apcha-oai@users.noreply.github.com>