Advertise shell snapshot v2 support on Unix (#40376)

## What changed

Set the local exec server's `shellSnapshotV2` capability on Unix, while
continuing to report it as unsupported on other platforms.

GitOrigin-RevId: 5e60cfb459c9312a3c56f020c11f7eece6643ab2

codex-rs/exec-server-protocol/src/protocol.rs

@@ -194,7 +194,7 @@ impl EnvironmentInfo {
                 environment_config_read: true,
                 http_header_env_vars: true,
                 sandboxed_file_streaming: true,
-                shell_snapshot_v2: false,
+                shell_snapshot_v2: cfg!(unix),
             },
         }
     }