fix: walk past an empty-string cursor in the legacy task refresh (#2220)

`ManagedRequestorTasksState.refresh()` drives the legacy `tasks/list` walk
itself, and both of its cursor tests were truthiness checks — so the fixed
adapter was never reached. `while (cursor)` ended the walk on a `""`
`nextCursor`, leaving the user-visible task list on page one, and
`cursor ? [...this.tasks, ...page] : page` would have replaced the
accumulated list rather than extended it had the walk continued.

Both now test `=== undefined`. The other four paged/managed stores already
did (`pagedToolsState`, `pagedPromptsState`, `pagedResourcesState`,
`pagedResourceTemplatesState`, `pagedRequestorTasksState`,
`managedSkillsState`); this was the last walk on the truthiness side.

Covered by a new case in `managedRequestorTasksState.test.ts` that paginates
`"" → "c2" → end` and asserts the cursor each call received, so a walk that
terminated correctly but re-requested page one still fails. Reverting either
guard on its own fails exactly that case.

Reported by Copilot on #2277.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EN2UfrkQZvtrviMtqURRm4
Signed-off-by: cliffhall <cliff@futurescale.com>

core/mcp/__tests__/fakeInspectorClient.ts

@@ -116,9 +116,12 @@ export class FakeInspectorClient
   listResourceTemplates = vi.fn(
     async () => this.resourceTemplatePages.shift() ?? { resourceTemplates: [] },
   );
-  listRequestorTasks = vi.fn(
-    async () => this.taskPages.shift() ?? { tasks: [] },
-  );
+  // Typed by its signature rather than inferred, so `mock.calls` carries the
+  // cursor a test asserts on — the empty-string pagination case (#2220) turns
+  // on *which* cursor each call received, not just how many there were.
+  listRequestorTasks = vi.fn<
+    (cursor?: string) => Promise<ListResult<"tasks", Task>>
+  >(async () => this.taskPages.shift() ?? { tasks: [] });
   listSkills = vi.fn(async () => this.skillPages.shift() ?? { skills: [] });
   // `skills/get` echoes a minimal entry; tests that care override the mock.
   getSkill = vi.fn(async (uri: string) => ({