Is your feature request related to a problem? Please describe.

This code:

async function foo(): Promise<string> {
  return "foo";
}
const result = await new MockActivityEnvironment().run(foo);
result.startsWith("foo");

Does not compile with strict types because result is inferred to be unknown instead of string.

Describe the solution you'd like

Change the signature of MockActivityEnvironment.run from:

run<P extends any[], R, F extends ActivityFunction<P, R>>(fn: F, ...args: P): Promise<R>;

to

run<F extends ActivityFunction>(fn: F, ...args: Parameters<F>): ReturnType<F>

I've checked this locally and it makes my example infer the type of result correctly.