fix: stop const-less anyOf branches from crashing the form, and never select a non-string enum

Closes #2007.

Two things, both surfaced while answering "does #1928's PR also fix #2007?".

**#2007, and why it belongs in this PR.** `z.array(z.union([z.object(…),
z.object(…)]))` gives an `items.anyOf` whose branches carry no top-level
`const`, so every MultiSelect option was `String(item.const ?? "")` — the
empty string, twice. Mantine *throws* on duplicate option values, greying
out the whole tool panel.

That is a pre-existing bug, but this branch makes it reachable from a new
direction: a *nullable* array of object unions previously had no top-level
`type` and fell to the JSON fallback, and now collapses to `type: "array"`
with `items.anyOf` — straight into the crash. Shipping the collapse without
this fix would widen an existing crash, so the two go together. Both shapes
are covered by tests.

`toConstOptions` now returns `null` unless every branch yields a distinct,
non-empty value, and the field falls through to the JSON editor — which is
the honest widget for a union of object shapes anyway. Applied to the
`string`/`oneOf` path as well, which had the identical `?? ""` and the same
duplicate-throw waiting on it.

**Copilot round 2.** Round 1's fix kept "an explicit `type` is
authoritative", so `{ type: "number", enum: [1, 2] }` still collapses with
its enum hoisted. The TUI's dispatcher tests `property.enum` *before* the
type switch, and `toSelectOptions` stringifies, so that reached a select
that submits `"1"` for `1`. The select path is now gated on `isStringEnum`
(exported from core for this), and a typed non-string enum falls through to
its typed field: the enum constraint is lost, the value's type is not,
which is the safer of the two losses. Pre-existing for a plain numeric
enum; the collapse just made the nullable form reachable too.

Signed-off-by: cliffhall <cliff@futurescale.com>

clients/tui/src/utils/schemaToForm.ts

@@ -3,7 +3,10 @@
  */
 
 import type { FormStructure, FormSection, FormField } from "ink-form";
-import { normalizeNullableUnion } from "@inspector/core/json/nullableUnion.js";
+import {
+  isStringEnum,
+  normalizeNullableUnion,
+} from "@inspector/core/json/nullableUnion.js";
 
 /** Minimal JSON Schema property shape used when building tool parameter forms */
 interface JsonSchemaProperty {
@@ -105,12 +108,17 @@ export function schemaToForm(
         ...baseField,
         options: toSelectOptions(property.items.enum, property.items.enumNames),
       } as FormField;
-    } else if (property.enum) {
-      // Single select
+    } else if (isStringEnum(property.enum)) {
+      // Single select. Gated on the members being strings because
+      // `toSelectOptions` stringifies them and ink-form hands the string
+      // straight back: a numeric `enum: [1, 2]` would submit `"1"` and violate
+      // the schema. A typed non-string enum falls through to its typed field
+      // below, which loses the enum constraint but keeps the value's type —
+      // the safer of the two losses.
       field = {
         type: "select",
         ...baseField,
-        options: toSelectOptions(property.enum, property.enumNames),
+        options: toSelectOptions(property.enum ?? [], property.enumNames),
       } as FormField;
     } else {
       // Map JSON Schema types to ink-form types