replace chatbot dataset search with semantic indexer api

[JHZhang2736]

What this PR does

1. Add new Semantic Search API client
2. Replace Search Datasets implementation
   • Use the semantic-indexer-api instead of the original search API.
3. Display chunk text in results
   • PDF files: Display only the most relevant chunk.
   • CSV files: First call /retrieve to get all chunks, then display them.
4. Support file format filtering
   • Allow user to specify file format (e.g., CSV or PDF) in queries.
   • Example: "Find some CSV files about endangered species in Australia".