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>

AGENTS.md

@@ -55,7 +55,15 @@ v2/main/
 │   │                                   #   annotation scan/validation + mirrored-param
 │   │                                   #   derivation, used by the Tools tab — #1632;
 │   │                                   #   plus `Mcp-Param-*` header building for the
-│   │                                   #   wire, used by both `tools/call` paths — #1846)
+│   │                                   #   wire, used by both `tools/call` paths — #1846;
+│   │                                   #   nullableUnion.ts: collapses the two JSON Schema
+│   │                                   #   encodings of "T or null" (`anyOf` with a null
+│   │                                   #   branch, `type: [T,"null"]`) into a plain type,
+│   │                                   #   hoisting the surviving branch's own keywords.
+│   │                                   #   Shared by BOTH form builders — web SchemaForm
+│   │                                   #   and TUI schemaToForm — since each dispatches on
+│   │                                   #   a single `type` string and would otherwise miss
+│   │                                   #   a nullable field entirely — #1928/#2015)
 │   ├── logging/                        # Silent pino logger singleton
 │   ├── mcp/                            # InspectorClient runtime + state stores
 │   │                                   #   (modernTaskSchemas.ts: SEP-2663 modern Tasks