feat(api): add non-streaming keep-alive support for idle timeout prevention

- Introduced `StartNonStreamingKeepAlive` to emit periodic blank lines during non-streaming responses.
- Added `nonstream-keepalive` configuration option in `SDKConfig`.
- Updated handlers to utilize `StartNonStreamingKeepAlive` and ensure proper cleanup.
- Extended config diff and tests to include `nonstream-keepalive` changes.

config.example.yaml

@@ -77,6 +77,9 @@ routing:
 # When true, enable authentication for the WebSocket API (/v1/ws).
 ws-auth: false
 
+# When true, emit blank lines every 5s for non-streaming responses to prevent idle timeouts.
+nonstream-keepalive: false
+
 # Streaming behavior (SSE keep-alives + safe bootstrap retries).
 # streaming:
 #   keepalive-seconds: 15   # Default: 0 (disabled). <= 0 disables keep-alives.