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

Beginne damit, die Diskussion im referenzierten Pull Request zu prüfen und den Code zu finden, der den nextLink/after-Wert abruft. Bestätige die aktuelle Behandlung von Klammern und überprüfe anschließend, dass Werte in Klammern extrahiert werden, während Werte ohne Klammern alle relevanten Informationen behalten.

Vom Indexierungsmodell aus dem Issue-Text verfasst.