fuzztest-rust | format duration flags as seconds.

By default, humantime::Duration formats durations as composite human-readable strings (such as "1m 30s"). However, this format is not compatible with absl::Duration. Hence we convert the duration to seconds while emitting the flag.

PiperOrigin-RevId: 964730385

rust/src/options.rs

diff --git a/rust/src/options.rs b/rust/src/options.rs
index 053cd81..0879705 100644
--- a/rust/src/options.rs
+++ b/rust/src/options.rs
@@ -212,10 +212,11 @@
                         // not specifying `--stop_after` means to run indefinitely.
                     }
                     FuzzFor::Duration(duration) => {
+                        let duration_secs = duration.as_secs_f64();
                         if opt_corpusdb.is_some() {
-                            add_arg(format!("--fuzztest_time_limit_per_test={duration}"))?;
+                            add_arg(format!("--fuzztest_time_limit_per_test={duration_secs}s"))?;
                         } else {
-                            add_arg(format!("--stop_after={duration}"))?;
+                            add_arg(format!("--stop_after={duration_secs}s"))?;
                         }
                     }
                 }