chore(react-call): silence biome warnings on async mutation primitive

Two leftover warnings after the feat commit:

1. mutate.test.tsx: `type NoProps = void` was matching the lib's
   `Props = void` default to exercise the void-Props ergonomic form.
   biome's `noConfusingVoidType` doesn't know that context — same
   `biome-ignore` reason is already used in callable-types.test.ts.

2. createCallable/index.tsx: the `storedMutationFn!(...)` non-null
   assertion was correct (the early-return guard above proves it
   defined), but TypeScript doesn't narrow the `let` across the
   closure boundary. Capture into a `const fn` after the guard so
   the closure sees the narrowed type and no assertion is needed.

No behaviour change.

Author: desko27

packages/react-call/src/__tests__/mutate.test.tsx

@@ -379,6 +379,7 @@ describe('CallOptions plumbing', () => {
   })
 
   test('void Props: call(options) form (no props arg) routes options correctly', async () => {
+    // biome-ignore lint/suspicious/noConfusingVoidType: matching the lib's `Props = void` default to exercise the void-Props ergonomic form
     type NoProps = void
     const NoPropsComp: ReactCall.UserComponent<
       NoProps,

packages/react-call/src/createCallable/index.tsx

@@ -122,12 +122,16 @@ export function createCallable<
         return
       }
 
+      // Capture the narrowed value into a const so the closure can
+      // call it without a non-null assertion — the early-return guard
+      // above proved storedMutationFn is defined.
+      const fn = storedMutationFn
       const mutationContext: MutationContext<Response> = {
         end: createEnd(promise),
       }
 
       Promise.resolve()
-        .then(() => storedMutationFn!(mutationContext, payload))
+        .then(() => fn(mutationContext, payload))
         .catch(() => {
           // Swallow — the caller's own try/catch inside mutationFn handles
           // UI side-effects (toasts, alerts, etc.). The dialog stays open