fix: concurrent map write for migrating schedules back to v1 (#10164)

## What changed?

Some observed panics in production spotted by @yycptt were causing
crashes. The mutation `searchattribute.AddSearchAttribute(&sa,
sadefs.TemporalNamespaceDivision,
payload.EncodeString(legacyscheduler.NamespaceDivision))` presumably
intermittently was happening while another read was happening elsewhere
in the system, causing a crash.

## Why?
Need to avoid panics in prod

## How did you test it?
- [X] built
- [ ] run locally and tested manually
- [X] covered by existing tests
- [ ] added new unit test(s)
- [ ] added new functional test(s)

## Potential risks
Not expected to be high risk as it's just a clone, might need some more
test coverage however. I am unsure if we have enough coverage to ensure
there's no data drops

Author: davidporter-id-au

chasm/lib/scheduler/scheduler_migrate_task.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"maps"
 	"time"
 
 	"github.com/google/uuid"
@@ -152,7 +153,7 @@ func (h *SchedulerMigrateToWorkflowTaskHandler) Execute(
 	// Build the start request to match createScheduleWorkflow in the frontend
 	// as closely as possible. Include TemporalNamespaceDivision so the V1
 	// workflow is discoverable via ListSchedules.
-	sa := &commonpb.SearchAttributes{IndexedFields: result.searchAttributes}
+	sa := &commonpb.SearchAttributes{IndexedFields: maps.Clone(result.searchAttributes)}
 	searchattribute.AddSearchAttribute(&sa, sadefs.TemporalNamespaceDivision, payload.EncodeString(legacyscheduler.NamespaceDivision))
 	workflowID := legacyscheduler.WorkflowIDPrefix + result.scheduleID
 	startReq := &workflowservice.StartWorkflowExecutionRequest{
@@ -165,7 +166,7 @@ func (h *SchedulerMigrateToWorkflowTaskHandler) Execute(
 		Identity:                 fmt.Sprintf("temporal-scheduler-migration-%s-%s", result.namespace, result.scheduleID),
 		WorkflowIdReusePolicy:    enumspb.WORKFLOW_ID_REUSE_POLICY_ALLOW_DUPLICATE,
 		WorkflowIdConflictPolicy: enumspb.WORKFLOW_ID_CONFLICT_POLICY_FAIL,
-		Memo:                     &commonpb.Memo{Fields: result.memo},
+		Memo:                     &commonpb.Memo{Fields: maps.Clone(result.memo)},
 		SearchAttributes:         sa,
 		Priority:                 &commonpb.Priority{},
 	}