Mock actor setters do not chain

[dahlia]

Background

MockFederation.setActorDispatcher() in packages/testing/src/mock.ts returns an object whose setKeyPairsDispatcher(), mapHandle(), mapAlias(), and authorize() return the MockFederation itself instead of the setters object. Other mock setters, such as those returned by setObjectDispatcher() and the collection dispatchers, return the federation from authorize() in the same way.

So code that chains setters, as the Federation API allows, fails against the mock:

federation
  .setActorDispatcher("/users/{identifier}", dispatcher)
  .setKeyPairsDispatcher(keyPairsDispatcher)
  .mapHandle(mapper);  // TypeError: mapHandle is not a function

mapPortableActorId(), added in #840, already returns the setters object, as does the real implementation. The actor setters also lack mapActorAlias().

Proposed work

Make every setters object that the mock returns return itself from its methods, and add the missing mapActorAlias().

Tests

• chaining every actor setter in any order;
• chaining authorize() on object and collection setters;
• the registered callbacks still being used by the mock context.

[xqliu]

Hi, I'd like to take this one if it's still open.

I have a fix: every setters object the mock returns (actor, object, and the collection dispatchers) now returns itself, the actor setters gain mapActorAlias(), and there are tests for the three cases listed above. Since this is a bug, I'd send the chaining fix against 2.0-maintenance and mapActorAlias() (added in 2.3) separately against 2.3-maintenance. Or would you prefer something else?

Per AI_POLICY.md: the code was written with AI tools (Orbi, an AI coding agent, and Claude Code). I've read the diff and run the tests myself.