WebDAV MKCOL: return 405 Exists if directory already exists (RFC 4918)

P3 fix:
- create_dir: check vfs.exists() before creating
- Return FsError::Exists (405 Method Not Allowed) if path exists

Tests: 36 webdav tests pass

markbase-core/src/webdav.rs

@@ -645,6 +645,9 @@ impl DavFileSystem for VfsDavFs {
             Ok(p) => p,
             Err(e) => return Box::pin(std::future::ready(Err(e))),
         };
+        if self.vfs.exists(&full_path) {
+            return Box::pin(std::future::ready(Err(FsError::Exists)));
+        }
         match self.vfs.create_dir_all(&full_path, 0o755) {
             Ok(_) => Box::pin(std::future::ready(Ok(()))),
             Err(_) => Box::pin(std::future::ready(Err(FsError::NotImplemented))),