op-supervisor: Treat reaching the safe head as a success not a failure. (#10982)

Current fault dispute rules are that the safe head is extended to the end of the trace.

op-program/client/driver/program.go

 package driver
 
 import (
-	"errors"
 	"fmt"
 
 	"github.com/ethereum/go-ethereum/log"
@@ -54,13 +53,13 @@ func (d *ProgramDeriver) OnEvent(ev rollup.Event) {
 		d.Emitter.Emit(engine.PendingSafeRequestEvent{})
 	case engine.ForkchoiceUpdateEvent:
 		if x.SafeL2Head.Number >= d.targetBlockNum {
-			d.logger.Info("Derivation complete: reached L2 block", "head", x.UnsafeL2Head)
+			d.logger.Info("Derivation complete: reached L2 block", "head", x.SafeL2Head)
 			d.closing = true
 		}
 	case derive.DeriverIdleEvent:
 		// Not enough data to reach target
 		d.closing = true
-		d.result = errors.New("not enough data to reach target")
+		d.logger.Info("Derivation complete: no further data to process")
 	case rollup.ResetEvent:
 		d.closing = true
 		d.result = fmt.Errorf("unexpected reset error: %w", x.Err)

op-program/client/driver/program_test.go

@@ -103,13 +103,13 @@ func TestProgramDeriver(t *testing.T) {
 			require.NoError(t, p.result)
 		})
 	})
-	// on exhaustion of input data: stop with error
+	// on exhaustion of input data: stop without error
 	t.Run("deriver idle", func(t *testing.T) {
 		p, m := newProgram(t, 1000)
 		p.OnEvent(derive.DeriverIdleEvent{})
 		m.AssertExpectations(t)
 		require.True(t, p.closing)
-		require.NotNil(t, p.result)
+		require.Nil(t, p.result)
 	})
 	// on inconsistent chain data: stop with error
 	t.Run("reset event", func(t *testing.T) {