feat(agent): export StreamingError to public API

StreamingError was not re-exported despite being the error type in
StreamingResult. This prevented users from pattern matching on
error variants or implementing From<StreamingError> for custom
error types when building multi-provider abstractions.

Signed-off-by: Jiawei Zhao <Phoenix500526@163.com>

Author: Phoenix500526

rig/rig-core/src/agent/mod.rs

@@ -115,7 +115,7 @@ pub use crate::message::Text;
 pub use builder::{AgentBuilder, AgentBuilderSimple};
 pub use completion::Agent;
 pub use prompt_request::streaming::{
-    FinalResponse, MultiTurnStreamItem, StreamingPromptRequest, stream_to_stdout,
+    FinalResponse, MultiTurnStreamItem, StreamingError, StreamingPromptRequest, stream_to_stdout,
 };
 pub use prompt_request::{CancelSignal, PromptRequest, PromptResponse};
 pub use prompt_request::{PromptHook, StreamingPromptHook};