Add comprehensive fuzz testing infrastructure

Co-authored-by: imjasonh <210737+imjasonh@users.noreply.github.com>

fuzz/fuzz_targets/parser.rs

@@ -0,0 +1,48 @@
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+use testscript_rs::parser;
+
+fuzz_target!(|data: &[u8]| {
+    // Convert bytes to string, handling invalid UTF-8 gracefully
+    let input = String::from_utf8_lossy(data);
+    
+    // Fuzz the main parser function
+    // The parser should never panic and always return a proper Result
+    let result = parser::parse(&input);
+    
+    // We don't care if parsing succeeds or fails, just that it doesn't panic
+    // If it succeeds, verify the result is well-formed
+    if let Ok(ref script) = result {
+        // Basic sanity checks - these should never fail for valid parsed scripts
+        // This helps catch internal consistency bugs
+        for command in &script.commands {
+            // Command name should not be empty if command exists
+            assert!(!command.name.is_empty(), "Empty command name");
+            // Line number should be positive
+            assert!(command.line_num > 0, "Invalid line number");
+        }
+        
+        for file in &script.files {
+            // File name should not be empty if file exists
+            assert!(!file.name.is_empty(), "Empty file name");
+        }
+    }
+    
+    // Test that the parser is deterministic - same input should produce same result
+    let result2 = parser::parse(&input);
+    match (result.is_ok(), result2.is_ok()) {
+        (true, true) => {
+            // Both succeeded - results should be identical
+            if let (Ok(ref script1), Ok(ref script2)) = (&result, &result2) {
+                assert_eq!(script1, script2, "Parser is not deterministic");
+            }
+        }
+        (false, false) => {
+            // Both failed - this is fine, errors don't need to be identical
+        }
+        _ => {
+            panic!("Parser is not deterministic - one call succeeded, other failed");
+        }
+    }
+});