Review follow-up: _cmd_tools rebuilt its listing without
state.agent.disabled_toolsets, so a config-disabled toolset was filtered
from execution but still advertised by /tools. Pass it through, matching
the session tool-surface rebuild.
New tests exercise the real get_tool_definitions (no patching) and assert
a disabled toolset is absent from both the /tools listing and the rebuilt
valid_tool_names — with a baseline assertion that the toolset is present
when nothing is disabled, so the check cannot pass vacuously.
d2a073f93b