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>

core/json/nullableUnion.ts

@@ -53,8 +53,7 @@ function toBranch(value: unknown): Record<string, unknown> | null {
 }
 
 /**
- * Whether a branch's `enum` is one a *typeless* branch can be read as a string
- * enum.
+ * Whether an `enum` is one that can be rendered as a string-valued dropdown.
  *
  * JSON Schema's `enum` is untyped — `[1, 2]`, `[true, false]`, and `[null]` are
  * all legal — so a bare `{ enum: [...] }` does **not** imply strings. Guessing
@@ -63,8 +62,12 @@ function toBranch(value: unknown): Record<string, unknown> | null {
  * would `String(...)` them and submit `"1"` where the server expects `1`. So
  * only an all-string enum earns the inference; anything else stays on the
  * fallback path, which renders the value honestly as JSON.
+ *
+ * Exported because the same question decides whether a *dispatcher* may route
+ * an enum to a select at all. The TUI's does, and its options are stringified,
+ * so an unguarded numeric enum there submits `"1"` where the server wants `1`.
  */
-function isStringEnum(value: unknown): boolean {
+export function isStringEnum(value: unknown): boolean {
   return (
     Array.isArray(value) &&
     value.length > 0 &&