fix: address Copilot review on the _meta widening

- Widen the three `_meta` annotations the bulk pass missed, all of them
  return types rather than identifier declarations: `mergeMeta`,
  `aggregateListParams`, and the `tools/call` params object. The second was
  unsound in a way callers could see — it advertised
  `{ _meta: Record<string, string> }` while the runtime carried nested JSON.

- `normalizeStoredMetadata`: keep an explicit `null` from a legacy pair
  instead of folding it into `""` (only a missing `value` defaults now), and
  store through `Object.defineProperty` so a legacy `__proto__` key becomes an
  own property rather than hitting the prototype setter.

- Drop the unjustified `as unknown as` from the migration test; the legacy
  fixture is untyped and cast once where it crosses into the typed parser.

- The TUI settings test asserted only that the app mounted, so it could not
  have caught a pair array being forwarded as numeric `_meta` keys. The fake
  client now captures its constructor options and the test asserts the nested
  object arrives intact.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0177mPHAdECD18nLLCTwR5rh
Signed-off-by: cliffhall <cliff@futurescale.com>

core/mcp/inspectorClient.ts

@@ -1037,7 +1037,7 @@ export class InspectorClient extends InspectorClientEventTarget {
    */
   private mergeMeta(
     callMetadata?: RequestMetadata,
-  ): Record<string, string> | undefined {
+  ): RequestMetadata | undefined {
     const defaults = this.defaultMetadata;
     // Modern-era per-request log level (#1629): stamp the opt-in `_meta` key on
     // every request so the server emits `notifications/message` on this
@@ -1132,7 +1132,7 @@ export class InspectorClient extends InspectorClientEventTarget {
    */
   private aggregateListParams(
     metadata?: RequestMetadata,
-  ): { _meta: Record<string, string> } | undefined {
+  ): { _meta: RequestMetadata } | undefined {
     const effectiveMeta = this.mergeMeta(metadata);
     return effectiveMeta ? { _meta: effectiveMeta } : undefined;
   }
@@ -4279,7 +4279,7 @@ export class InspectorClient extends InspectorClientEventTarget {
     const callParams: {
       name: string;
       arguments: Record<string, JsonValue>;
-      _meta?: Record<string, string>;
+      _meta?: RequestMetadata;
       task?: { ttl: number };
     } = {
       name: tool.name,

core/mcp/serverList.ts

@@ -234,7 +234,19 @@ export function normalizeStoredMetadata(
       }
       const { key, value } = entry as { key: string; value?: JsonValue };
       if (key.trim() === "") continue;
-      out[key] = value ?? "";
+      // `??`, not `||`: `null` is a legal `_meta` value and must survive the
+      // migration as `null`. Only a genuinely absent `value` becomes `""`.
+      //
+      // `defineProperty` rather than `out[key] = …` because a legacy key of
+      // `__proto__` would otherwise hit the prototype setter — silently
+      // dropping the entry (or, for an object value, mutating the prototype)
+      // instead of storing an own property under that name.
+      Object.defineProperty(out, key, {
+        value: value === undefined ? "" : value,
+        writable: true,
+        enumerable: true,
+        configurable: true,
+      });
     }
     return out;
   }