Update sdk run as sandbox

[Dingway98]

Note

Medium Risk
Medium risk because it changes how process commands are serialized (quoting/flattening args and dropping use_shell) and adds runAs propagation across runtime file/process APIs, which could affect command execution and permissions behavior.

Overview
Adds first-class user impersonation support to sandbox runtime calls by introducing run_as/runAs on SandboxExecParams, threading it through SandboxHandle.exec, and providing SandboxFilesApi.with_run_as() to apply runAs to all file requests.

Refactors process execution to accept either a command string or SandboxExecParams plus optional overrides (cwd, env, timeouts, run_as), normalizing legacy fields by shell-quoting and folding args into command while omitting args/useShell from the wire payload.

Updates E2E/contract tests to use the new with_run_as("root") pattern, to accept create events in file watching assertions, and to add retry/backoff when creating sandboxes from newly-created snapshots; bumps SDK version to 0.90.1.

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