Stronger types and better error handling in executeMock (#7370)

This brings the `execute_mock` function into line with the `execute` function, which I tweaked in https://github.com/KittyCAD/modeling-app/pull/7351. Now mock execution, like real execution, will always return a properly-formatted KCL error, instead of any possible JS value.

Also, incidentally, I noticed that send_response always succeeds, so I changed it from Result<()> to void.

rust/kcl-lib/src/execution/typed_path.rs

@@ -220,6 +220,7 @@ impl schemars::JsonSchema for TypedPath {
 ///
 /// * Does **not** touch `..` or symlinks – call `canonicalize()` if you need that.
 /// * Returns an owned `PathBuf` only when normalisation was required.
+#[cfg(not(target_arch = "wasm32"))]
 fn normalise_import<S: AsRef<str>>(raw: S) -> std::path::PathBuf {
     let s = raw.as_ref();
     // On Unix we need to swap `\` → `/`.  On Windows we leave it alone.