fix(lint): add more return statements after t.Fatal for SA5011

Continue fixing staticcheck SA5011 warnings about potential nil pointer
dereference by adding return statements after t.Fatal calls.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: technicalpickles

internal/config/merge_test.go

@@ -404,6 +404,7 @@ func TestMergeServerConfig_ConfigDiffCapture(t *testing.T) {
 
 	if diff == nil {
 		t.Fatal("Expected diff to be generated")
+		return // unreachable but satisfies staticcheck SA5011
 	}
 
 	// Check modified fields
@@ -727,6 +728,7 @@ func TestMergeServerConfig_ComplexNestedMerge(t *testing.T) {
 	// Verify diff captured expected changes
 	if diff == nil {
 		t.Fatal("Expected diff to be generated")
+		return // unreachable but satisfies staticcheck SA5011
 	}
 	if len(diff.Modified) < 5 {
 		t.Errorf("Expected at least 5 modified fields, got %d", len(diff.Modified))

internal/updatecheck/checker_test.go

@@ -28,6 +28,7 @@ func TestChecker_CheckNow(t *testing.T) {
 
 	if info == nil {
 		t.Fatal("CheckNow returned nil")
+		return // unreachable but satisfies staticcheck SA5011
 	}
 
 	if info.CurrentVersion != "v1.0.0" {