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>

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

@@ -0,0 +1,67 @@
+{
+  "serverInfo": {
+    "name": "root-union-schemas-showcase",
+    "version": "1.0.0"
+  },
+  "tools": [{ "preset": "echo" }, { "preset": "get_weather" }],
+  "rawToolSchemas": {
+    "echo": {
+      "inputSchema": {
+        "type": "object",
+        "properties": {
+          "message": { "type": "string" }
+        },
+        "required": ["message"],
+        "anyOf": [
+          {
+            "type": "object",
+            "properties": {
+              "kind": { "type": "string", "const": "email" },
+              "address": { "type": "string" }
+            },
+            "required": ["kind", "address"]
+          },
+          {
+            "type": "object",
+            "properties": {
+              "kind": { "type": "string", "const": "sms" },
+              "phone": { "type": "string" }
+            },
+            "required": ["kind", "phone"]
+          }
+        ]
+      }
+    },
+    "get_weather": {
+      "inputSchema": {
+        "type": "object",
+        "discriminator": { "propertyName": "by" },
+        "oneOf": [
+          {
+            "type": "object",
+            "title": "By city",
+            "properties": {
+              "by": { "type": "string", "const": "city" },
+              "city": { "type": "string" }
+            },
+            "required": ["by", "city"]
+          },
+          {
+            "type": "object",
+            "title": "By coordinates",
+            "properties": {
+              "by": { "type": "string", "const": "coords" },
+              "lat": { "type": "number" },
+              "lon": { "type": "number" }
+            },
+            "required": ["by", "lat", "lon"]
+          }
+        ]
+      }
+    }
+  },
+  "transport": {
+    "type": "streamable-http",
+    "port": 6604
+  }
+}