fix: strip the enum null sentinel, and make the preset test cover its wiring

Copilot round 4: no new visible comments, two in the suppressed block. Both
valid.

1. The `type: [T, "null"]` encoding keeps its keywords at the top level, so a
   nullable enum written that way carries the null *inside* the list:
   `{ type: ["string", "null"], enum: ["envio", "recebimento", null] }`. The
   collapse left it there, which broke both renderers in different ways — the
   web dispatcher handed `null` to Mantine as option data, and the TUI's
   all-strings check rejected the whole enum and fell back to a plain text
   field. Neither is the dropdown the schema asks for.

   The collapse now strips a `null` member whenever it flattens, since that
   fact has already moved onto `nullable`. An enum of nothing but `null`
   drops out entirely: it offers no value a dropdown could show, so the plain
   widget serves the field better than an empty select. Covered in core and
   end-to-end in both dispatchers.

2. The new integration test claimed in its own header to pin the registry and
   config wiring, then called `createNullableFieldsTool()` directly — so a
   misspelt preset case or a config naming a dead preset would have left it
   green. A fair "this does not test what it says" catch. It now resolves the
   checked-in `nullable-fields-http.json` through `loadConfig` →
   `resolveConfig`, matching `duplicate-tool-names.test.ts`, and asserts the
   resolved tool names before booting the server.

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

clients/web/src/components/groups/SchemaForm/SchemaForm.test.tsx

@@ -886,6 +886,34 @@ describe("SchemaForm nullable unions", () => {
     ).not.toBeInTheDocument();
   });
 
+  // The other supported nullable encoding: keywords stay at the top level, so
+  // the null sentinel sits inside the enum list rather than on a branch.
+  it("renders a Select for a type: [string, null] enum, without a null option", async () => {
+    const user = userEvent.setup();
+    const onChange = vi.fn();
+    const schema: InspectorFormSchema = {
+      type: "object",
+      properties: {
+        direction: {
+          title: "Direction",
+          type: ["string", "null"],
+          enum: ["envio", "recebimento", null] as unknown as string[],
+        },
+      },
+    };
+    renderWithMantine(
+      <SchemaForm schema={schema} values={{}} onChange={onChange} />,
+    );
+    await user.click(screen.getByRole("textbox", { name: "Direction" }));
+    const options = await screen.findAllByRole("option", { hidden: true });
+    expect(options.map((option) => option.textContent)).toEqual([
+      "envio",
+      "recebimento",
+    ]);
+    await user.click(options[0]);
+    expect(onChange).toHaveBeenCalledWith({ direction: "envio" });
+  });
+
   it("renders a TextInput for a type: [string, null] field", async () => {
     const user = userEvent.setup();
     const onChange = vi.fn();