plugin.run and plug-in commands read `ids` strictly: a value that is not
a list, or an entry that is not a non-negative integer, is an error
naming `ids` ("ids must be an array of non-negative integer object
ids"). Left out or null, `ids` is still the selection.
A new test shows that a plug-in command that runs out of its
instruction budget leaves the document and the undo history unchanged,
and that the error names the plug-in and the command.
docs/plugins.md and docs/mcp.md name MCP's `command_list` and
`list_commands` as the listings that show plug-in commands, and the
params text, docs and progress line describe the `ids` rule.