fix: reject empty queries in search and embed endpoints

[echobt]

This PR adds validation to the /search, /embed, and /embed_batch endpoints to reject empty or whitespace-only inputs.

Changes

• Modified search handler to return 400 Bad Request if query is empty or whitespace.
• Modified embed handler to return 400 Bad Request if text is empty or whitespace.
• Modified embed_batch handler to return 400 Bad Request if the list is empty or contains only empty strings.
• Added logic verification tests in src/server/api_tests.rs.

Reason

Empty queries waste resources by generating embeddings and searching the database for meaningless input. This validation prevents unnecessary computation and returns a clear error to the client.