Add sandbox storage_mb support

[mstolarzblaxelai]

• Adds storage_mb to sandbox create config and serialized runtime models.
• Forwards root storage through async and sync sandbox creation plus image builds.
• Covers SDK serialization and build payload behavior.

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Note

Adds storage_mb as an optional parameter to sandbox creation (async, sync, and image builds). The field flows from SandboxCreateConfiguration through to the SandboxRuntime model, serialized as storageMb in the API payload. Tests cover serialization and forwarding in both async and sync paths.

^{Written by Mendral for commit 0fb52ac.}

[mendral-app]

LGTM

Clean, minimal addition that follows the existing patterns for optional runtime fields. The None vs UNSET handling is correct, serialization round-trips through both snake_case and camelCase keys, and tests cover the key paths. No correctness, security, or data issues found.

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