nit(#154): add t.Fatal guard if baseSubprocessArgs flag not found

Address sonnet NIT: if --repo or --pr is ever removed from
baseSubprocessArgs(), the mutation loop silently no-ops and the test
becomes meaningless. Adding a found guard and t.Fatal makes the
regression immediately visible.

cmd/review-bot/main_test.go

@@ -903,12 +903,17 @@ func TestMainSubprocess_InvalidRepo(t *testing.T) {
 		flag.CommandLine = flag.NewFlagSet(os.Args[0], flag.ExitOnError)
 		args := baseSubprocessArgs()
 		// Replace the canonical --repo value with an invalid one.
+		found := false
 		for i, a := range args {
 			if a == "--repo" && i+1 < len(args) {
 				args[i+1] = "invalidrepo"
+				found = true
 				break
 			}
 		}
+		if !found {
+			t.Fatal("baseSubprocessArgs() does not contain --repo; test is broken")
+		}
 		os.Args = args
 		main()
 		return
@@ -930,12 +935,17 @@ func TestMainSubprocess_InvalidPRNumber(t *testing.T) {
 		flag.CommandLine = flag.NewFlagSet(os.Args[0], flag.ExitOnError)
 		args := baseSubprocessArgs()
 		// Replace the canonical --pr value with a non-numeric string.
+		found := false
 		for i, a := range args {
 			if a == "--pr" && i+1 < len(args) {
 				args[i+1] = "notanumber"
+				found = true
 				break
 			}
 		}
+		if !found {
+			t.Fatal("baseSubprocessArgs() does not contain --pr; test is broken")
+		}
 		os.Args = args
 		main()
 		return