[Feature Request] Hiding "Knowledge Detail Settings" when "Use your existing knowledge base" is selected

## [Request]
When creating a new bot, users can still input "Knowledge Detail Settings" even after selecting "Use your existing knowledge base."
In reality, settings such as the embedding model and advanced parsing of the existing Knowledge base cannot be modified.
To avoid user confusion, I would like to hide the "Knowledge Detail Settings" section when "Use your existing knowledge base" is selected.

## [References]
https://github.com/aws-samples/bedrock-claude-chat/blob/57c55b4067e588b64cd6d9a9fea1949c484a5e1e/frontend/src/features/knowledgeBase/pages/BotKbEditPage.tsx#L1857-L1863

Open frontend/src/features/knowledgeBase/pages/BotKbEditPage.tsx around lines 1857-1863 and inspect how the new-bot form tracks the existing knowledge base selection. Verify the Knowledge Detail Settings section is hidden for that selection and remains available otherwise; confirm the form still renders correctly in both states.

Written by the indexing model from the issue text.