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>

test-servers/configs/nullable-fields-http.json

@@ -0,0 +1,11 @@
+{
+  "serverInfo": {
+    "name": "nullable-fields-showcase",
+    "version": "1.0.0"
+  },
+  "tools": [{ "preset": "record_shipment" }, { "preset": "get_temp" }],
+  "transport": {
+    "type": "streamable-http",
+    "port": 6602
+  }
+}