fix sandbox create idempotency key

[kcoopermiller]

Note

Medium Risk
Changes how create generates and persists idempotency_key across transient failures; if incorrect, could cause unintended duplicate sandbox creation or prevent legitimate retries. Scope is limited to sandbox create payload construction and associated tests.

Overview
Fixes sandbox creation idempotency so automatic idempotency keys are stable across a failed create retry.

CreateSandboxRequest now stores a private _generated_idempotency_key, and sync/async SandboxClient.create() uses a helper to reuse the same generated key until a successful response clears it (while still honoring user-supplied idempotency_key). Adds coverage to ensure key reuse after a failure and key rotation on subsequent successful creates.

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Reset generated key when idempotency key is empty

_create_idempotency_key treats request.idempotency_key as absent when it is an empty string (""), but _clear_generated_create_idempotency_key only clears when it is exactly None. In that input case, a generated key is reused across all later successful create calls with the same request object, so distinct creates can be deduplicated unexpectedly and return the earlier sandbox. Align the clear condition with the generation condition (or validate non-empty idempotency keys) to avoid sticky keys.

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