[Security Solution] [Enhancement] The system prompt marked as default should be retained if no new system prompt is marked as Default

[muskangulati-qasource]

Describe the bug
The system prompt marked as default should be retained if no new system prompt is marked as Default

Kibana/Elasticsearch Stack version

VERSION: 8.18.0 BC7
BUILD: 82701
COMMIT: 9df464f0ca6c78d2119bbef0b32953d6ba5ff4fe

Steps

1. Kibana version 8.18.0 BC7 should exist
2. Navigate to Stack Management --> AI Assistant --> Security AI settings --> System Prompts
3. Add a system prompt and make it default and save the same
4. Add a new prompt, make it default, uncheck the default checkbox
5. Save the prompt
6. Observe, the default prompt before adding the new prompt is not retained.

Expected Result
If user cancels an operation, the default configuration should go back to original system prompt

Screen Recording

SystemPrompts.mp4

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)

[muskangulati-qasource]

@arvindersingh-qasource please review!

[elasticmachine]

Pinging @elastic/security-entity-analytics (Team:Entity Analytics)