fix: address Copilot review round 27 — unconstrained null branches, typed discriminators

- anyOfAdmitsNull recognizes a branch that constrains nothing (`{}`, or
  annotations only): it admits every value, null among them, so a field pinned
  to `const: null` beside such a union stays submittable. `admitsNull`
  cannot say that on its own — an unconstrained schema tells it nothing either
  way and it declines rather than guesses — but here the question is narrower.
- convertToolParameters sends a matched `const` as the schema's own typed
  value: `kind=2` selects a branch pinned to `const: 2` and was then sent
  `"2"`, which that same branch rejects. Only an exact match is
  substituted; anything else is the user's input and is left alone.

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

core/json/jsonUtils.ts

@@ -264,7 +264,15 @@ export function convertToolParameters(
   for (const [key, value] of Object.entries(params)) {
     const paramSchema = properties[key] as ParameterSchema | undefined;
 
-    if (paramSchema) {
+    // A `const` the supplied text names is sent as the schema's own typed
+    // value, not as the text: a branch pinned to `const: 2` is selected by
+    // `kind=2` and would otherwise be sent `"2"`, which that same branch
+    // rejects. Only an exact match is substituted — anything else is the
+    // user's input and is left alone.
+    const pinned = (paramSchema as { const?: unknown } | undefined)?.const;
+    if (pinned !== undefined && String(pinned) === value) {
+      result[key] = pinned as JsonValue;
+    } else if (paramSchema) {
       result[key] = convertParameterValue(value, paramSchema);
     } else {
       result[key] = value;