[red-knot] Don't check non-python files (#17021)

## Summary

Fixes https://github.com/astral-sh/ruff/issues/17018

## Test Plan

I renamed a python file to `knot.toml` and verified that there are no
diagnostics. Renaming back the file to `*.py` brings back the
diagnostics

crates/red_knot_wasm/src/lib.rs

@@ -214,6 +214,10 @@ impl FileHandle {
     pub fn js_to_string(&self) -> String {
         format!("file(id: {:?}, path: {})", self.file, self.path)
     }
+
+    pub fn path(&self) -> String {
+        self.path.to_string()
+    }
 }
 
 #[wasm_bindgen]