fix: keep terminal helper script alive until child exits

[alexsmolya]

terminal-helper deletes the generated temporary script after the terminal launcher returns, but kgx --wait does not guarantee that delayed pkexec/bash has already consumed it. This can produce failures such as bash: /tmp/tmp...: No such file or directory.

This change:

• gives the generated script normal cleanup ownership through an EXIT trap while preserving the child exit status;
• keeps terminal-launch failure cleanup parent-owned;
• leaves terminal detection/order, pkexec flow, rootshell behavior, GUI logic, and gaming package command construction unchanged;
• adds deterministic coverage for delayed child startup, direct bash, the rootshell/fake-pkexec path, non-zero child exit, and terminal-launch failure.

The tests use harmless marker commands only; no real root, package, or system mutation is used.

Validation:

• bash -n src/scripts/terminal-helper src/scripts/rootshell.sh tests/terminal-helper_test.sh
• tests/terminal-helper_test.sh
• git diff --check

Fixes #287

[alexsmolya]

I'm continuing this work as a partner, not as a competitor. The existing terminal-helper fix and its investigation may provide useful evidence, so I want to preserve and validate that work rather than throw it away.