fix(raftadmin): fakeEngine implements RegisterLeaderAcquiredCallback

CI failure on PR #723 (test (ubuntu-latest)):
TestServerMapsEngineAdminMethods returned "add voter is not
supported by this raft engine" because the type-assertion at
internal/raftadmin/server.go:21 (any(engine).(raftengine.Admin))
silently fell back to admin=nil after Phase 3.D PR 4-B-3b
extended the Admin interface with RegisterLeaderAcquiredCallback.
The test stub fakeEngine never gained the method, so it stopped
satisfying the interface — and every admin RPC mapped through
NewServer returned Unimplemented.

This is exactly the "semantic-change-without-full-caller-audit"
miss the recent lessons-learned discipline targets. PR #723's
audit grep was on the production resolver helpers, not on the
Admin interface itself; raftadmin's stub slipped through.

Fix: add the missing method on the stub. Returns a no-op
deregister — raftadmin doesn't exercise the leader-acquired
observer (PR 4-B-3b's SQS leadership-refusal lives in
main_sqs_leadership_refusal.go), the stub's only obligation is
to satisfy the type assertion.

go test -race ./... and golangci-lint clean.

Author: bootjp

internal/raftadmin/server_test.go

@@ -137,6 +137,18 @@ func (f *fakeEngine) TransferLeadership(context.Context) error {
 	return nil
 }
 
+func (f *fakeEngine) RegisterLeaderAcquiredCallback(fn func()) func() {
+	// raftadmin doesn't exercise the leader-acquired observer (PR
+	// 4-B-3b's SQS leadership-refusal hook lives in main_sqs_*).
+	// The stub satisfies the raftengine.Admin interface so the
+	// type-assertion in NewServer succeeds; without this method
+	// the assertion would silently fall back to admin=nil and every
+	// admin RPC would return Unimplemented (regression caught by
+	// TestServerMapsEngineAdminMethods after Phase 3.D PR 4-B-3b
+	// extended the Admin interface).
+	return func() {}
+}
+
 func (f *fakeEngine) TransferLeadershipToServer(_ context.Context, id string, address string) error {
 	f.mu.Lock()
 	defer f.mu.Unlock()