Settings can now offer a fixed list of choices instead of free text

What

• Config-style setting fields can now declare a fixed options array (1-64 character strings, no control or bidirectional characters, no leading/trailing spaces, case-insensitive-unique), for string-type, non-multiple, non-sensitive fields; a default among the options or required: true must be set.
• MCP server config forms now render such a field as a select field with options and a hint, instead of always showing free text.
• Plugin-provided string settings that declare an options array now render as a pick-to-commit enum/choice control instead of free text.

Why

This lets settings with a fixed set of valid values (for MCP servers or plugins) be presented as a picker instead of a free-text box, reducing invalid input.