fix: handle sandbox create_if_not_exists races

[mstolarzblaxelai]

• Thread create_if_not_exist through sandbox and CodeInterpreter create paths.
• Use the server-side create-if-not-exist path with bounded 409 retry handling for non-reusable sandbox states.
• Part of ENG-2188; supersedes feat: add createIfNotExist query param support in SDK #120.

---

Note

Refactors create_if_not_exists to thread a create_if_not_exist flag through to the server-side API, then adds a bounded 3-attempt retry loop to handle races where a sandbox is found in a non-reusable state (TERMINATED, FAILED, etc.) between the 409 response and the next create attempt. Shared helpers _is_sandbox_conflict, _sandbox_name, and NON_REUSABLE_SANDBOX_STATUSES are extracted to eliminate duplication between async and sync paths.

^{Written by Mendral for commit 6bfe6c8.}

[mendral-app]

LGTM

Logic is correct: the loop correctly handles the create→409→get→non-reusable→retry cycle, and the 3-attempt ceiling prevents infinite loops. No security, data-loss, or correctness issues found. The only non-critical gap is the absence of any backoff between retry attempts — under sustained contention (e.g., a slow backend deletion), all 3 attempts may fire within milliseconds and exhaust before the terminal sandbox is cleaned up, but this is a reliability trade-off rather than a correctness bug.

_{Tag @mendral-app with feedback or questions. View session}