fix: render nullable enum fields as a Select instead of a raw JSON textarea

A tool argument that is optional AND explicitly nullable — Zod's
`.nullish()` / `.nullable().optional()` — compiles to
`anyOf: [{ type: "string", enum: [...] }, { type: "null" }]`. Every branch
of the tool-call form's field dispatcher tests a single `type` string, so
such a field matched nothing and fell through to the raw-JSON fallback,
whose onChange stores the unparseable in-progress text back into state and
re-`JSON.stringify`s it on the next render — compounding a fresh layer of
escaping on every keystroke.

`normalizeUnionType` already existed for the FastMCP `string|null` shape,
but it only copied the surviving branch's `type` and dropped everything
else, so the `enum` never reached the dispatcher; and nothing called it on
a tool input schema — only elicitation requests, via
`resolveRefsInMessage`. So:

- Rewrite `normalizeUnionType` to hoist the non-null branch's own keywords
  (`enum`, `items`, `properties`, `minimum`, …) onto the flattened schema,
  as v1.x did. This also collapses nine near-identical blocks into two and
  picks up the `object`/`array` cases the `type: [X, "null"]` form was
  missing.
- Normalize each field in `SchemaForm.renderField`, so tool, elicitation,
  and app forms all get it rather than just elicitation.
- Give a nullable enum `Select` a clear affordance, so a value can be set
  back to `null`; a non-nullable enum keeps none, since it would reject it.

Adds a `record_shipment` preset and `nullable-fields-http.json` showcase
config serving all four nullable scalar shapes, for manual verification.

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

README.md

@@ -145,6 +145,7 @@ Each config below is a ready-made server for exercising one feature by hand. Loa
 | `pagination-http.json`                    | Page-by-page list fetching                         | [#1721](https://github.com/modelcontextprotocol/inspector/issues/1721) |
 | `structured-output-http.json`             | Tools tab: a result's `structuredContent` section  | [#1908](https://github.com/modelcontextprotocol/inspector/issues/1908) |
 | `duplicate-tool-names-http.json`          | A `tools/list` that repeats a tool name            | [#1957](https://github.com/modelcontextprotocol/inspector/issues/1957) |
+| `nullable-fields-http.json`               | Tools tab: nullable (`anyOf` + `null`) arguments   | [#1928](https://github.com/modelcontextprotocol/inspector/issues/1928) |
 | `advertised-extensions-http.json`         | Tool registration gated on advertised extensions   | [#1739](https://github.com/modelcontextprotocol/inspector/issues/1739) |
 | `logging-{legacy,modern}-http.json`       | Logging, both eras                                 | [#1629](https://github.com/modelcontextprotocol/inspector/issues/1629) |
 | `subscriptions-{legacy,modern}-http.json` | Resource subscriptions, both eras                  | [#1630](https://github.com/modelcontextprotocol/inspector/issues/1630) |
@@ -227,6 +228,12 @@ Connect (default legacy era), open the Tools tab, and type `get` into **Search t
 
 The duplicated copies are appended rather than placed beside their twin on purpose. React matches a leading run of same-key children first, so a head-adjacent duplicate happens to line up and the defect hides; separating the pair is what makes it observable — and it is also the realistic shape, two tool sources concatenated.
 
+#### Nullable arguments
+
+`nullable-fields-http.json` serves `record_shipment`, whose four arguments are each declared with Zod's `.nullish()` — "optional **and** explicitly nullable". That compiles to `anyOf: [<branch>, { "type": "null" }]`, so the real type (and, for the enum, its `enum` list) sits on a branch rather than at the top level. `get_temp` sits alongside it with a plain, non-nullable `units` enum for comparison. Plain streamable-HTTP — connect with the **default (legacy)** protocol era.
+
+Open the Tools tab and select `record_shipment`: `direction` must render as a **Select** (`envio` / `recebimento`) with a clear button that sets it back to `null`, `reference` as a text input, `quantity` as a number input, and `express` as a checkbox. On the broken build every one of them fell through to the raw-JSON textarea, which re-escaped its own contents on each keystroke until the value was unusable ([#1928](https://github.com/modelcontextprotocol/inspector/issues/1928)). The tool echoes the arguments it received, so the result panel shows exactly what was sent.
+
 #### Advertised extensions
 
 `advertised-extensions-http.json` serves `echo` (always) and a `get_weather` tool **gated on the `io.modelcontextprotocol/tasks` extension** (`extensionGatedTools`): the tool is registered but starts disabled, and the server enables it on `notifications/initialized` only when the client declared that extension in its `capabilities.extensions`.