Update sdk run as sandbox

[Dingway98]

Note

Medium Risk
Adds privilege-selection (runAs) to sandbox file and process operations and changes how process commands are serialized, which could affect runtime behavior/permissions and backward compatibility of process invocation.

Overview
Adds runAs support across sandbox APIs. SandboxFilesApi now supports withRunAs() to set a default user and automatically injects runAs into all file-related query/body payloads.

Updates process execution ergonomics and payload normalization. SandboxProcessesApi.exec/start gain string overloads with SandboxExecOptions, always send a single command string plus runAs (dropping args/useShell from the runtime payload), and the sandbox handle exec now forwards string+options to the process API.

Docs/tests are updated accordingly (README example, new process API unit tests, e2e files tests run as root by default, snapshot lifecycle tests add settle delay/retry), and the package version bumps to 0.89.1.

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