fix(tools): surface non-zero bash exits as tool errors

[PsiACE]

• BUB_MAX_STEPS=5
• BUB_API_FORMAT=responses
• BUB_MODEL=openai:openai/gpt-5.4

Before Fix

result=EXCEPTION: max_steps_reached=5
tool_result={'results': ['boom']}
step=1 status=continue
step=2 status=continue
step=3 status=continue
step=4 status=continue
step=5 status=continue

After Fix

result=The bash tool reported an execution failure: the command exited with code 7 and output `boom`. Per
your instruction, I’m stopping here.
tool_result={'results': [{'kind': 'tool', 'message': "Tool 'bash' execution failed.", 'details': {'error':
"RuntimeError('command exited with code 7\\noutput:\\nboom')"}}]}
step=1 status=continue
step=2 status=ok