fix: decline a union branch requiring a name it never declares (#2224)

`isOfferable` in `core/json/rootUnion.ts` checked that a merged branch carries
at least one property and that every property value is readable, but never that
each name in `required` resolves to a property a form can render. A branch like
`{ properties: { kind }, required: ["kind", "payload"] }` was therefore offered
in the picker, rendered no control for `payload`, and had the submit-time check
report it missing permanently — the exact dead end the function's own comment
already reasons through for a `false`-schema required field, one case short.

Judge the merge, so a branch requiring only names the ROOT declares —
`anyOf: [{ required: ["email"] }, …]` — stays offerable as before. `hasOwn`
rather than `in`, since an argument legally named `constructor` would otherwise
resolve through the prototype and read as declared.

Declining every branch of a union leaves the web form with neither a picker nor
a field, which is a different dead end. `SchemaForm` now seeds the raw-JSON
editor (#2151) in that one case — a root union that resolved to no branches and
no properties — so the arguments stay expressible. Seeded rather than forced, so
the switch still works, and re-seeded during render (never in an effect) because
the form is reused across tools rather than remounted. A tool that genuinely
takes no arguments is untouched. The TUI has no such editor; there the union is
simply not offered, which is already better than a section that can never be
submitted.

Adds a `record_shipment_by` preset and wires it into the
`root-union-schemas-http.json` showcase so the shape is reproducible by hand,
with docs/test-servers.md covering it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PxYT4eKdnx1yRw7x4bW7Ts
Signed-off-by: cliffhall <cliff@futurescale.com>

test-servers/src/test-server-fixtures.ts

@@ -272,6 +272,35 @@ export function createGetWeatherTool(): ToolDefinition {
   };
 }
 
+/**
+ * Create a tool whose advertised arguments are a root `oneOf` **no branch of
+ * which can be offered**, for #2224. Each branch requires a name it never
+ * declares, so a form builder that enumerates `properties` alone renders no
+ * control for it while the submit gate reports it missing forever.
+ *
+ * The Zod `inputSchema` here is a placeholder: the shape that matters is the
+ * raw JSON Schema `root-union-schemas-http.json` substitutes through
+ * `rawToolSchemas`, since Zod cannot emit a root composition. The handler is
+ * what a real server would do with the arguments — echo them back, so what the
+ * form actually sent is visible in the result.
+ */
+export function createDeadEndUnionTool(): ToolDefinition {
+  return {
+    name: "record_shipment_by",
+    description:
+      "Record a shipment, by address or by tracking number. Its advertised schema is a root oneOf whose branches each require a name they never declare, so no branch is renderable.",
+    inputSchema: {
+      by: z
+        .string()
+        .optional()
+        .describe("Which alternative the call is making"),
+    },
+    handler: async (params: Record<string, unknown>) => {
+      return toToolResult(`Recorded shipment: ${JSON.stringify(params)}`);
+    },
+  };
+}
+
 /**
  * Create a tool whose SEP-2243 `x-mcp-header` annotation is INVALID: the header
  * name `"Bad Header"` contains a space, so it is not a valid RFC 9110 token.