refactor: align GenerateSearchIndicatorEvents return type with other event builders

Change GenerateSearchIndicatorEvents to return [][]byte instead of []sseEvent
for consistency with BuildFallbackTextEvents and other event building functions.

Benefits:
- Consistent API across all event generation functions
- Eliminates intermediate sseEvent type conversion in caller
- Simplifies usage by returning ready-to-send SSE byte slices

This addresses the code quality feedback from PR #226 review.

internal/runtime/executor/kiro_executor.go

@@ -4487,7 +4487,7 @@ func (e *KiroExecutor) handleWebSearchStream(
 				select {
 				case <-ctx.Done():
 					return
-				case out <- cliproxyexecutor.StreamChunk{Payload: []byte(event.ToSSEString())}:
+				case out <- cliproxyexecutor.StreamChunk{Payload: event}:
 				}
 			}
 			contentBlockIndex += 2