feat: add OpenSandbox as secure code execution backend

[hellomypastor]

Describe the changes you have made:

Integrate [OpenSandbox](https://github.com/alibaba/OpenSandbox) as an optional secure code execution backend. When enabled, Python, Shell, and JavaScript code runs inside isolated containers instead of directly on the host machine.

New files:

• interpreter/core/computer/terminal/languages/sandbox/sandbox_manager.py — Manages OpenSandbox lifecycle: lazy sandbox creation, per-language execution contexts, streaming output via queue-based generator (mirrors SubprocessLanguage pattern), interrupt support, and cleanup.
• interpreter/core/computer/terminal/languages/sandbox/sandbox_language.py — SandboxLanguage base class extending BaseLanguage, delegates execution to the shared SandboxManager.
• interpreter/core/computer/terminal/languages/sandbox/__init__.py — Concrete subclasses: SandboxPython, SandboxShell, SandboxJavaScript.

Modified files:

• terminal.py — Conditionally swaps Python/Shell/JavaScript handlers with sandbox variants when sandbox_mode=True; adds sandbox-aware instantiation in _streaming_run; skips local import_computer_api in sandbox mode; calls sandbox_manager.terminate() on cleanup.
• core.py — Adds sandbox_mode, sandbox_api_key, sandbox_domain constructor parameters.
• start_terminal_interface.py — Adds --sandbox, --sandbox_api_key, --sandbox_domain CLI arguments.
• pyproject.toml — Adds opensandbox + opensandbox-code-interpreter as optional [sandbox] extras.

Usage:

# CLI
interpreter --sandbox --sandbox_api_key "your-key" --sandbox_domain "localhost:8080"

# Python API
interpreter.sandbox_mode = True
interpreter.sandbox_api_key = "your-key"
interpreter.sandbox_domain = "localhost:8080"

Design decisions:

• Opt-in only — sandbox_mode defaults to False, zero impact on existing behavior.
• Shared sandbox instance — A single SandboxManager owns one sandbox container; multiple languages share it via separate execution contexts, minimizing resource overhead.
• Lazy initialization — Sandbox is created on first code execution, not at startup.
• Graceful degradation — Unsupported languages (Ruby, R, AppleScript, etc.) continue using local execution even in sandbox mode. Missing SDK packages produce a clear ImportError message.

Reference any relevant issues (e.g. "Fixes #000"):

N/A

Pre-Submission Checklist (optional but appreciated):

• I have included relevant documentation updates (stored in /docs)
• I have read docs/CONTRIBUTING.md
• I have read docs/ROADMAP.md

OS Tests (optional but appreciated):

• Tested on Windows
• Tested on MacOS
• Tested on Linux