test(api/federation): register mock.AssertExpectations on federationHandler() mock #44

Problem

federationHandler() in internal/api/handler_federation_test.go creates a MockAuthService with a single ValidateSession stub but never registers t.Cleanup(func() { mockAuth.AssertExpectations(t) }). All 40+ federation tests that call federationHandler() inherit this gap.

Per the project feedback rule (feedback_mock_assert_expectations.md): every mock must have AssertExpectations registered via t.Cleanup so unmatched On stubs are caught as test failures rather than silently passing.

Fix

Refactor federationHandler() to accept a *testing.T parameter, register t.Cleanup(func() { mockAuth.AssertExpectations(t) }) inside it, and update all call sites. Since the same mockAuth is used for every federation test, this one change covers the entire file.

Discovered during adversarial review of PR LeanerCloud/cloud-commitments-cli#890 (was pre-existing, not introduced by that PR).