[minutechreview]

terminalFailureNotice only treated a missing sandbox as idle when the message matched not found. Providers now throw SandboxNotFoundError, whose message is "The sandbox does not exist", so an expired sandbox printed a red [UseAgent] The sandbox does not exist line in the terminal pane.

• backend/src/runs/terminal.ts: check for SandboxNotFoundError (by instanceof, or by error.name in case the error crosses a package copy) before the text check, and return the "no live sandbox yet" notice.
• backend/src/runs/terminal.test.ts: replace the hand-written new Error("Sandbox abc not found") input with new SandboxNotFoundError(), and assert the notice is not the red error line. The existing "connection timed out" case still prints as an error.

Testing:

• bun test src/runs/terminal.test.ts: 2 pass. With terminal.ts from main, the updated case fails with Received: "\r\n\u001B[31m[UseAgent] The sandbox does not exist\u001B[0m\r\n".
• backend tsc --noEmit (TypeScript 5.9.3) is clean.

Fixes #313

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