test(uffd): restore TestParallelMissing/Write parallelOperations to 1_000_000

Reverts the 1M -> 10K reduction made in #2461. Intermittent failures
under 1M are bugs to be hunted, not silenced by lowering the load.
The other parallel/serial variants in these files have legitimately
been at 10K since their introduction in November 2025; only the two
TestParallelMissing(/Write) cases were reduced.

Author: ValentaTomas

packages/orchestrator/pkg/sandbox/uffd/userfaultfd/missing_test.go

@@ -141,7 +141,7 @@ func TestMissing(t *testing.T) {
 func TestParallelMissing(t *testing.T) {
 	t.Parallel()
 
-	parallelOperations := 10_000
+	parallelOperations := 1_000_000
 
 	tt := testConfig{
 		pagesize:      header.PageSize,

packages/orchestrator/pkg/sandbox/uffd/userfaultfd/missing_write_test.go

@@ -136,7 +136,7 @@ func TestMissingWrite(t *testing.T) {
 func TestParallelMissingWrite(t *testing.T) {
 	t.Parallel()
 
-	parallelOperations := 10_000
+	parallelOperations := 1_000_000
 
 	tt := testConfig{
 		pagesize:      header.PageSize,