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

首先查看所引用的 pull request 讨论，并定位获取 nextLink/after 值的代码。确认当前的方括号处理方式，然后验证方括号内的值会被提取，而不带方括号的值会保留所有相关信息。

由索引模型根据 Issue 内容生成。