fix: centralize the nullable-union collapse in core and apply it in the TUI

Closes #2015, folded into #1928 rather than left as a follow-up.

The TUI's `schemaToForm` has the same defect the web `SchemaForm` had: its
local `JsonSchemaProperty` declares only `type?: string` — no `anyOf`, no
array-valued `type` — so a `.nullish()` argument has no top-level `type` or
`enum` for the dispatcher to read and falls to the `switch` default, losing
its select. Milder than the web bug (a plain text field, not a corrupting
JSON textarea), but the same class.

Fixing it in a second place is the argument for not having the logic in a
first place. So the collapse moves to `core/json/nullableUnion.ts` as
`normalizeNullableUnion`, generic over a minimal `NullableUnionSchema`
shape — the least a caller must expose to be *recognized* as a nullable
union — so neither client has to adopt the other's schema type. Both form
builders now share one copy and cannot drift on which schemas they render.

- `core/json/nullableUnion.ts` — the shared collapse, plus tests at
  `clients/web/src/test/core/nullableUnion.test.ts` (the web suite is what
  gates `core/`).
- `clients/web`: `schemaUtils.normalizeUnionType` is gone; `SchemaForm` and
  `resolveRefsInMessage` call the core function directly.
- `clients/tui`: `schemaToForm` normalizes each property before dispatch,
  and `JsonSchemaProperty` grows the `anyOf` / array-`type` fields the
  encoding uses.

Verified by driving the built TUI under a pty against the
`nullable-fields-http.json` showcase server. Before, every argument of
`record_shipment` rendered as a plain text field; after, `direction` is a
select ("No value"), `quantity` an integer, `express` a boolean
("[Not set]").

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

clients/tui/src/utils/schemaToForm.ts

@@ -3,10 +3,16 @@
  */
 
 import type { FormStructure, FormSection, FormField } from "ink-form";
+import { normalizeNullableUnion } from "@inspector/core/json/nullableUnion.js";
 
 /** Minimal JSON Schema property shape used when building tool parameter forms */
 interface JsonSchemaProperty {
-  type?: string;
+  /**
+   * An array here is the `["string", "null"]` nullable encoding, which
+   * {@link normalizeNullableUnion} collapses to a single name before the
+   * dispatch below reads it.
+   */
+  type?: string | string[];
   title?: string;
   enum?: unknown[];
   /** Non-standard legacy support: titles for enum values */
@@ -15,6 +21,8 @@ interface JsonSchemaProperty {
   minimum?: number;
   maximum?: number;
   default?: unknown;
+  /** Present on a nullable union; see {@link normalizeNullableUnion}. */
+  anyOf?: readonly unknown[];
 }
 
 /**
@@ -68,9 +76,16 @@ export function schemaToForm(
     // `properties` values are `unknown` (the SDK schema admits anything), so
     // guard before treating a value as a schema object — a malformed server
     // schema with e.g. `properties: { foo: null }` must not throw on `.title`.
-    const property = (
-      typeof prop === "object" && prop !== null ? prop : {}
-    ) as JsonSchemaProperty;
+    // Flatten a nullable union (`anyOf: [X, {type:"null"}]`, `type: [X,"null"]`)
+    // before dispatching. Every branch below reads a single `type` string, and
+    // for an enum the `enum` keyword sits on the union's surviving branch — so
+    // without this an argument declared with Zod's `.nullish()` loses its
+    // select and degrades to a plain text field (#2015, the TUI twin of #1928).
+    const property = normalizeNullableUnion(
+      (typeof prop === "object" && prop !== null
+        ? prop
+        : {}) as JsonSchemaProperty,
+    );
     const baseField = {
       name: key,
       label: property.title || key,