saveSession: strip a file:// prefix like loadSession does

[juliusbangert]

loadSession accepts an Expo/RN-style file:// URI and strips it before calling native; saveSession passes the string straight through, and llama_state_save_file fopen()s it as given, so the save fails with "Failed to save session". Same on main (src/index.ts):

async loadSession(filepath: string): Promise<NativeSessionLoadResult> {
  const { llamaLoadSession } = getJsi()
  let path = filepath
  if (path.startsWith('file://')) path = path.slice(7)
  return llamaLoadSession(this.id, path)
}

async saveSession(filepath: string, options?: { tokenSize: number }): Promise<number> {
  const { llamaSaveSession } = getJsi()
  return llamaSaveSession(this.id, filepath, options?.tokenSize || -1)
}

Every other path-taking method in the file (loadSession, the multimodal and TTS ones) strips the prefix; saveSession is the odd one out. This makes it consistent:

   async saveSession(
     filepath: string,
     options?: { tokenSize: number },
   ): Promise<number> {
     const { llamaSaveSession } = getJsi()
-    return llamaSaveSession(this.id, filepath, options?.tokenSize || -1)
+    let path = filepath
+    if (path.startsWith('file://')) path = path.slice(7)
+    return llamaSaveSession(this.id, path, options?.tokenSize || -1)
   }

We hit it passing new File(Paths.cache, name).uri from expo-file-system, which is a file:// URI; loadSession on the same value works.