Update how nextLink value is retrieved

Currently the nextLink/after value is retrieved by removing the brackets and saving the contents. However, it is done by assuming that the brackets are in the first and last positions of the string. We need to update it to ensure that we do not delete any relevant information in case the brackets are not there.

Solutions:
1) Using `IndexOf` and `LastIndexOf` functions to check if the brackets exist and use their positions to delete them with regex (Similar to the way that it is done now)
2) Using only regex to only use the information found inside the brackets if they exist.

This task is made in reference to the following comment:
https://github.com/Azure/data-api-builder/pull/2893#discussion_r2413840792

まず、参照されている pull request のディスカッションを確認し、nextLink/after 値を取得するコードを見つけます。現在の角括弧の処理を確認し、そのうえで、角括弧で囲まれた値が抽出され、角括弧のない値では関連する情報がすべて保持されることを検証します。

索引モデルが issue の本文から書いたものです。