fix: render tool schemas with root-level anyOf/oneOf composition

A tool's `inputSchema` may carry composition keywords at its root — the
2026-07-28 revision makes this explicit — but both form builders enumerate
the root's `properties` and nothing else. A schema keeping its fields on
`allOf`/`oneOf`/`anyOf` branches therefore rendered no controls at all: no
branch picker, no fields, not even the raw-JSON fallback a union-typed
*property* falls back to, so the tool could only be called with empty
arguments.

Adds `core/json/rootUnion.ts`, one flattening shared by all three clients so
they cannot drift on which schemas they can render:

- `allOf` is merged unconditionally (conjunctive — there is no choice to
  present), a root `oneOf`/`anyOf` is returned as branches.
- Web `SchemaForm` renders a Variant picker above the fields and prunes the
  outgoing branch's values on a switch, since they are no longer visible and
  would describe a shape the call is not making.
- TUI `schemaToForm` gives each branch its own section with optional fields —
  ink-form is static, and only one alternative applies to a call.
- `convertToolParameters` looks through the branches, so a CLI `--tool-arg`
  is typed by the schema that declares it rather than sent as a string.

Required-field gating asks whether *any* branch is satisfied, which is
selection-independent and never blocks arguments the schema accepts. A
`const` is seeded like a `default` and rendered read-only: it admits exactly
one value, so anything else the user could type produces a rejected call.

Adds a `root-union-schemas-http.json` showcase server.

Closes #2123

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

clients/web/src/utils/toolUtils.test.ts

@@ -86,3 +86,39 @@ describe("toolRowKey / findToolByRowKey", () => {
     expect(findToolByRowKey(tools, "1:get_weather")).toBeUndefined();
   });
 });
+
+describe("hasInputFields with root composition (#2123)", () => {
+  const tool = (inputSchema: Tool["inputSchema"]): Tool => ({
+    name: "t",
+    inputSchema,
+  });
+
+  it("sees fields declared on a root union branch", () => {
+    expect(
+      hasInputFields(
+        tool({
+          type: "object",
+          anyOf: [
+            { type: "object", properties: { a: { type: "string" } } },
+            { type: "object", properties: { b: { type: "string" } } },
+          ],
+        }),
+      ),
+    ).toBe(true);
+  });
+
+  it("sees fields declared on a root allOf", () => {
+    expect(
+      hasInputFields(
+        tool({
+          type: "object",
+          allOf: [{ type: "object", properties: { a: { type: "string" } } }],
+        }),
+      ),
+    ).toBe(true);
+  });
+
+  it("still reports no fields for a bare object schema", () => {
+    expect(hasInputFields(tool({ type: "object" }))).toBe(false);
+  });
+});