fix(board): exit when ostool-server releases session

[ZR233]

What changed

This updates remote board execution so ostool exits with an error when ostool-server releases the session or closes the serial websocket.

It applies the same handling to both direct board serial terminals and the remote U-Boot serial bridge used by ostool board.

Why

Previously, server-side closed and related websocket termination paths could be treated like a normal terminal shutdown on the client side.

That made ostool look like it ended cleanly even though the board session had actually been released by ostool-server.

Impact

When the server frees the board or tears down the serial websocket, the local ostool command now reports an error and exits.

This makes remote board session loss visible to users and to automation.

Validation

• cargo test -p ostool