Scripting: export commands a script runs directly respect Allow Scripts to Write Files (#2921)

* Scripting: export commands a script runs directly respect Allow Scripts to Write Files

CompItem.saveFrameToPng and exportAsMotionGraphicsTemplate checked the
file-write preference only in their JavaScript wrappers. A script could call
comp.saveFrameAs or essential.exportTemplate itself (app.run, __exec) and the
native handlers wrote the file with the preference off.

Session::refuse_export and write_output_with now refuse when the running
command was issued by a script and the preference is off, so every command
that writes a path it is given is gated the same way. The wrappers check the
gate with a new side-effect-free __file("checkWrite") instead of writing an
empty file at the destination.

* address review: clear stale export test dir before use

* test: allow script writes in script_imports_footage_and_saves

crates/host/src/tests.rs

@@ -547,6 +547,7 @@ fn script_imports_footage_and_saves() {
     std::fs::create_dir_all(&dir).unwrap();
     let q = |n: &str| serde_json::to_string(&dir.join(n).to_string_lossy()).unwrap();
     let mut s = super::session();
+    s.execute("prefs.set", json!({"key": "scripting.allowScriptsWriteFiles", "value": true})).unwrap();
     let code = format!(
         r#"
             var c = app.project.items.addComp("Src", 32, 32, 1, 1, 10);