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

Inizia esaminando la discussione della pull request indicata e individuando il codice che recupera il valore nextLink/after. Conferma la gestione attuale delle parentesi, quindi verifica che i valori tra parentesi vengano estratti, mentre i valori senza parentesi conservino tutte le informazioni pertinenti.

Scritto dal modello di indicizzazione a partire dal testo della issue.