Update with target config before invoking Centipede.

So that Centipede gets proper flags instead of the serialized target config. Makes it easier for people to understand/debug.

PiperOrigin-RevId: 933084230

centipede/centipede_flags.inc

diff --git a/centipede/centipede_flags.inc b/centipede/centipede_flags.inc
index 5dda892..f6d6da2 100644
--- a/centipede/centipede_flags.inc
+++ b/centipede/centipede_flags.inc
@@ -456,7 +456,9 @@
     std::string, fuzztest_configuration, "",
     "If set, deserializes the FuzzTest configuration from the value as a "
     "base64url string instead of querying the configuration via runner "
-    "callbacks. For FuzzTest framework only, do not use from end-users.")
+    "callbacks. Specially, if set to `(null)`, assumes no configuration "
+    "without querying the runner. For FuzzTest framework only, do not use from "
+    "end-users.")
 CENTIPEDE_FLAG(
     bool, list_crash_ids, false,
     "If set, lists the crash IDs of a single test of the binary to the "

centipede/centipede_interface.cc

diff --git a/centipede/centipede_interface.cc b/centipede/centipede_interface.cc
index 0a3917c..0131ab6 100644
--- a/centipede/centipede_interface.cc
+++ b/centipede/centipede_interface.cc
@@ -764,7 +764,9 @@
       // TODO: b/410051414 Use Centipede flags to pass necessary information
       // instead of passing the entirely serialized Configuration once switched
       // to the unified execution model.
-      if (!env.fuzztest_configuration.empty()) {
+      if (env.fuzztest_configuration == "(null)") {
+        return "";
+      } else if (!env.fuzztest_configuration.empty()) {
         std::string result;
         FUZZTEST_CHECK(
             absl::WebSafeBase64Unescape(env.fuzztest_configuration, &result));

fuzztest/internal/centipede_adaptor.cc

diff --git a/fuzztest/internal/centipede_adaptor.cc b/fuzztest/internal/centipede_adaptor.cc
index bfd67dc..aafc9e0 100644
--- a/fuzztest/internal/centipede_adaptor.cc
+++ b/fuzztest/internal/centipede_adaptor.cc
@@ -269,8 +269,8 @@
         std::string{test_name}};
     single_test_configuration.time_limit = total_time_limit;
     single_test_configuration.time_budget_type = TimeBudgetType::kTotal;
-    env.fuzztest_configuration =
-        absl::WebSafeBase64Escape(single_test_configuration.Serialize());
+    env.UpdateWithTargetConfig(single_test_configuration);
+    env.fuzztest_configuration = "(null)";
   }
 
   absl::StrAppend(&env.binary,