Fix issue #67: Retry / regenerate doesn't take you to new response

[onitbot]

This pull request fixes #67.

The changes directly address and resolve the reported issue through concrete code modifications:

1. The core fix adds logic in Model+Chat.swift to remove any responses that exist after the current generation index when regenerating a response. This is implemented through:

if prompt.responses.count > 0 {
    let currentIndex = prompt.generationIndex
    prompt.responses.removeSubrange((currentIndex + 1)...)
    prompt.priorInstructions.removeSubrange((currentIndex + 1)...)
}

2. The added test suite verifies this behavior by:

• Creating a prompt with 3 responses
• Setting the generation index to the middle response
• Triggering regeneration
• Confirming that subsequent responses are removed while preserving prior ones

The original issue occurred because old responses remained in the array after regeneration, causing navigation issues. With these changes, regenerating a response will:

• Remove any subsequent responses
• Add the new response at the current position
• Ensure the user sees the new response immediately since there are no later responses to navigate through

The implementation is complete and testable, with both the core fix and verification tests included. The changes directly solve the reported navigation problem by managing the response array properly during regeneration.

Automatic fix generated by OpenHands 🙌