refactor: optimize try_reserve by removing redundant overflow check

[AriajSarkar]

Description

This PR addresses the optimization suggested in #801 and implements the TODO left in #787.

It removes the redundant checked_add call in try_reserve. Since we are checking self.len() + additional <= self.capacity(), and additional is a legitimate increment for the internal storage, the overflow check is unnecessary and adding it was a defensive but redundant step.

Related Issues

• Fixes Optimize try_reserve by implementing TODO (remove redundant overflow check) #801
• Follow-up to fix: Prevent panic in try_reserve/try_with_capacity on capacity overflow #787