Fuzz target for the JavaScript engine's script runner (#6304)

Co-authored-by: eliets2 <287273517+eliets2@users.noreply.github.com>

crates/js/fuzz/Cargo.toml

@@ -0,0 +1,24 @@
+[package]
+name = "pdfcraft-js-fuzz"
+version = "0.0.0"
+publish = false
+edition = "2024"
+license = "MIT OR Apache-2.0"
+
+[package.metadata]
+cargo-fuzz = true
+
+# Detached from the PdfCraft workspace: run with `cargo +nightly fuzz run run_script`
+# from crates/js/.
+[workspace]
+
+[dependencies]
+libfuzzer-sys = "0.4"
+pdfcraft-js = { path = ".." }
+
+[[bin]]
+name = "run_script"
+path = "fuzz_targets/run_script.rs"
+test = false
+doc = false
+bench = false

crates/js/fuzz/fuzz_targets/run_script.rs

@@ -0,0 +1,26 @@
+//! Feeds arbitrary bytes, read as a script, to the JavaScript run entry with fixed event and document values.
+//!
+//! The run's result is discarded: the target only checks that no input panics or hangs past the limits.
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+use pdfcraft_js::{DocInfo, Event, Limits, run};
+
+/// The largest script the target runs (64 KiB); larger inputs are skipped.
+const MAX_INPUT: usize = 64 << 10;
+
+fuzz_target!(|data: &[u8]| {
+    if data.len() > MAX_INPUT {
+        return;
+    }
+    let script = String::from_utf8_lossy(data).into_owned();
+    let event = Event {
+        name: "Keystroke".into(),
+        kind: "Field".into(),
+        target: Some("field1".into()),
+        value: "value".into(),
+        change: String::new(),
+        will_commit: true,
+    };
+    let _outcome = run(&script, &event, &DocInfo::default(), &[], &[], Limits::default());
+});