TUI: support multi-select (array) fields in ink-form forms #2904

The TUI builds its tool and elicitation forms with schemaToForm (clients/tui/src/utils/schemaToForm.ts). ink-form has no multi-select, so a { type: "array", items: { enum: [...] } } property becomes a single select. That means:

• a tool argument that is an array of enums can only be sent with one value, and
• since TUI: handle sampling and elicitation requests from the server #2829 (PR TUI: handle sampling and elicitation requests from the server #2903), a multi-select elicitation field is answered with a one-element array. A required enum-array field with minItems: 2 cannot be satisfied, and the accept result goes out without its bounds checked.

Expected: the user can choose several values (a custom ink-form field manager, or a comma/JSON-array text input as a fallback), and minItems/maxItems are checked before the call or the elicitation answer is sent.

Raised by Copilot review on #2903 and declined there as out of scope: #2829 asked for the elicitation form to reuse the existing tool form, which has this limitation for tool arguments too.