Stabilize issue field merge order

Preserve deterministic issue field ordering when merging existing and incoming issue field values so update requests are stable.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: RossTarrant

pkg/github/issues.go

@@ -461,15 +461,27 @@ func fetchExistingIssueFieldValues(ctx context.Context, gqlClient *githubv4.Clie
 // mergeIssueFieldValues returns a merged slice where incoming values override existing ones
 // for the same field ID, and existing fields not present in incoming are preserved.
 func mergeIssueFieldValues(existing, incoming []*github.IssueRequestFieldValue) []*github.IssueRequestFieldValue {
-	merged := make(map[int64]*github.IssueRequestFieldValue, len(existing)+len(incoming))
+	indexByFieldID := make(map[int64]int, len(existing)+len(incoming))
+	result := make([]*github.IssueRequestFieldValue, 0, len(existing)+len(incoming))
 	for _, v := range existing {
-		merged[v.FieldID] = v
+		if v == nil {
+			continue
+		}
+		if _, ok := indexByFieldID[v.FieldID]; ok {
+			continue
+		}
+		indexByFieldID[v.FieldID] = len(result)
+		result = append(result, v)
 	}
 	for _, v := range incoming {
-		merged[v.FieldID] = v
-	}
-	result := make([]*github.IssueRequestFieldValue, 0, len(merged))
-	for _, v := range merged {
+		if v == nil {
+			continue
+		}
+		if index, ok := indexByFieldID[v.FieldID]; ok {
+			result[index] = v
+			continue
+		}
+		indexByFieldID[v.FieldID] = len(result)
 		result = append(result, v)
 	}
 	return result

pkg/github/issues_test.go

@@ -3182,6 +3182,27 @@ func Test_UpdateIssue(t *testing.T) {
 	}
 }
 
+func TestMergeIssueFieldValuesPreservesOrder(t *testing.T) {
+	existing := []*github.IssueRequestFieldValue{
+		{FieldID: 101, Value: "old priority"},
+		{FieldID: 102, Value: "Acme"},
+	}
+	incoming := []*github.IssueRequestFieldValue{
+		{FieldID: 101, Value: "P1"},
+		{FieldID: 103, Value: float64(7)},
+	}
+
+	merged := mergeIssueFieldValues(existing, incoming)
+
+	require.Len(t, merged, 3)
+	assert.Equal(t, int64(101), merged[0].FieldID)
+	assert.Equal(t, "P1", merged[0].Value)
+	assert.Equal(t, int64(102), merged[1].FieldID)
+	assert.Equal(t, "Acme", merged[1].Value)
+	assert.Equal(t, int64(103), merged[2].FieldID)
+	assert.Equal(t, float64(7), merged[2].Value)
+}
+
 func Test_ParseISOTimestamp(t *testing.T) {
 	tests := []struct {
 		name         string