Rename sandbox setting to chat.agent.sandbox (#303421)

Rename the top-level sandbox setting from `chat.tools.terminal.sandbox.enabled`
to `chat.agent.sandbox` to reflect that sandboxing is a general agent concept,
not terminal-specific.

- Update setting ID value to `chat.agent.sandbox`
- Update description to be more general
- Deprecate old `chat.tools.terminal.sandbox.enabled` setting
- Update telemetry event name

Fixes #303421

Co-authored-by: Copilot <[email protected]>