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/configs/root-union-schemas-http.json

@@ -3,7 +3,11 @@
     "name": "root-union-schemas-showcase",
     "version": "1.0.0"
   },
-  "tools": [{ "preset": "echo" }, { "preset": "get_weather" }],
+  "tools": [
+    { "preset": "echo" },
+    { "preset": "get_weather" },
+    { "preset": "record_shipment_by" }
+  ],
   "rawToolSchemas": {
     "echo": {
       "inputSchema": {
@@ -32,6 +36,26 @@
         ]
       }
     },
+    "record_shipment_by": {
+      "inputSchema": {
+        "type": "object",
+        "discriminator": { "propertyName": "by" },
+        "oneOf": [
+          {
+            "type": "object",
+            "title": "By address",
+            "properties": { "by": { "type": "string", "const": "address" } },
+            "required": ["by", "address"]
+          },
+          {
+            "type": "object",
+            "title": "By tracking number",
+            "properties": { "by": { "type": "string", "const": "tracking" } },
+            "required": ["by", "tracking"]
+          }
+        ]
+      }
+    },
     "get_weather": {
       "inputSchema": {
         "type": "object",