fix sandbox 409s

[kcoopermiller]

Note

Introduces targeted retries for transient 409 responses to improve reliability of gateway interactions.

• Adds MAX_409_RETRIES and RETRY_409_BASE_DELAY; implements _should_retry_409 (sync/async) that checks error-context and backs off when sandbox is RUNNING, otherwise raises the appropriate not-running error
• Wraps execute_command, upload_file/upload_bytes, and download_file (sync and async) in retry loops that specifically handle 409, preserve existing 408/timeout handling, and keep detailed error propagation
• Leaves existing gateway transient connection retry decorator intact for network-level errors

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