fix: address Copilot review round 41 — const does not demand a property, required-only branches

- `const` constrains a property that is PRESENT; it neither requires the
  property nor acts as a default. It is now supplied automatically only where
  the schema also requires the field — in `collectSchemaDefaults`,
  `applySchemaConstants` and the TUI's initial values alike — so an optional
  `dryRun: { const: true }` no longer turns a valid `{}` call into one that
  asks the server to do something. A value that IS present is still corrected
  to the constant, whatever supplied it, and the field stays read-only.
- Offerability is judged on the base+member merge rather than the raw member,
  so `anyOf: [{ required: ["email"] }, { required: ["phone"] }]` over
  root-declared properties is offered. It was declined, which left the gate
  checking the base alone and accepting `{}` — which that schema rejects.
- The TUI's branch sections keep the schema's own `required` for the seeding
  decision while still rendering every branch field non-required, which is what
  keeps a static form submittable.

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

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

@@ -375,15 +375,32 @@ describe("root composition (#2123)", () => {
     expect(collectSchemaDefaults(UNION)).not.toHaveProperty("phone");
   });
 
-  it("seeds a const property on an ordinary schema too", () => {
+  it("seeds a required const property on an ordinary schema too", () => {
     expect(
       collectSchemaDefaults({
         type: "object",
         properties: { version: { type: "string", const: "1" } },
+        required: ["version"],
       }),
     ).toEqual({ version: "1" });
   });
 
+  it("leaves an optional const out", () => {
+    // `const` constrains a present value; it does not require the property or
+    // act as a default, so seeding it would turn a valid `{}` call into one
+    // that asks the server to do something.
+    const optional: InspectorFormSchema = {
+      type: "object",
+      properties: { dryRun: { type: "boolean", const: true } },
+    };
+    expect(collectSchemaDefaults(optional)).toEqual({});
+    expect(applySchemaConstants(optional, {})).toEqual({});
+    // …but a value that IS present must be the one the schema fixes.
+    expect(applySchemaConstants(optional, { dryRun: false })).toEqual({
+      dryRun: true,
+    });
+  });
+
   it("prefers a const over a conflicting default", () => {
     // `default` is an annotation, not a constraint, so a schema may advertise
     // one its own `const` rejects — seeding it would submit an invalid value
@@ -394,6 +411,7 @@ describe("root composition (#2123)", () => {
         properties: {
           v: { type: "string", const: "a", default: "b" },
         },
+        required: ["v"],
       }),
     ).toEqual({ v: "a" });
   });
@@ -454,6 +472,7 @@ describe("root composition (#2123)", () => {
       properties: Object.fromEntries([
         ["__proto__", { type: "string", const: "kept" }],
       ]),
+      required: ["__proto__"],
     });
     expect(Object.hasOwn(seeded, "__proto__")).toBe(true);
   });
@@ -474,6 +493,7 @@ describe("root composition (#2123)", () => {
                   kind: { type: "string", const: "email" },
                   address: { type: "string" },
                 },
+                required: ["kind"],
               },
               {
                 type: "object",
@@ -481,6 +501,7 @@ describe("root composition (#2123)", () => {
                   kind: { type: "string", const: "sms" },
                   phone: { type: "string" },
                 },
+                required: ["kind"],
               },
             ],
           },