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/tui/__tests__/schemaToForm.test.ts

@@ -555,6 +555,7 @@ describe("schemaToForm", () => {
         {
           type: "object",
           properties: { v: { type: "string", const: "a", default: "b" } },
+          required: ["v"],
         },
         "const_tool",
       );
@@ -565,6 +566,26 @@ describe("schemaToForm", () => {
       });
     });
 
+    it("leaves an optional const unfilled", () => {
+      // `const` constrains a present value; it does not require the property
+      // or act as a default, so an optional one must stay omittable.
+      const form = schemaToForm(
+        {
+          type: "object",
+          properties: { dryRun: { type: "boolean", const: true } },
+        },
+        "optional_const",
+      );
+      const field = form.sections[0]!.fields[0] as {
+        initialValue?: unknown;
+        options?: unknown[];
+      };
+      expect(field.initialValue).toBeUndefined();
+      // The single option is still offered to a user who wants it.
+      expect(field.options).toEqual([{ label: "true", value: "true" }]);
+      expect(decodeFormValues({ type: "object" }, {})).toEqual({});
+    });
+
     it("renders a branch's specialization of a root property in its section", () => {
       const form = schemaToForm(
         {