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/components/groups/SchemaForm/SchemaForm.test.tsx

@@ -2344,6 +2344,7 @@ describe("SchemaForm multiline strings (#2042)", () => {
               a: { type: "string", title: "A" },
               version: { type: "string", const: "2" },
             },
+            required: ["version"],
           }}
           values={{}}
           onChange={onChange}
@@ -2402,6 +2403,7 @@ describe("SchemaForm multiline strings (#2042)", () => {
           kind: { type: "string", const: value },
           note: { type: "string", title: "Note" },
         },
+        required: ["kind"],
       });
       const { rerender } = renderWithMantine(
         <SchemaForm