feat(agent): export StreamingError to public API

[Phoenix500526]

Export StreamingError from the rig::agent module to enable proper error handling in multi-provider abstractions.

Motivation

When building applications that support multiple LLM providers, a common pattern is to create an enum wrapper to abstract over different model types:

  pub enum AgentBuilder {
      Anthropic(agent::AgentBuilder<anthropic::completion::CompletionModel>),
      OpenAI(agent::AgentBuilder<openai::completion::CompletionModel>),
      Gemini(agent::AgentBuilder<gemini::completion::CompletionModel>),
      OpenRouter(agent::AgentBuilder<openai::completion::CompletionModel>),
  }

To unify error handling across these providers, users need to define their own error types and implement conversions from the underlying errors:

  pub enum MyStreamingError {
      Streaming(StreamingError),
      // ...
  }

  impl From<StreamingError> for MyStreamingError {
      fn from(e: StreamingError) -> Self {
          MyStreamingError::Streaming(e)
      }
  }

However, StreamingError was not re-exported from the agent module, making this pattern impossible.

The Problem

StreamingResult is defined as:

  pub type StreamingResult<R> =
      Pin<Box<dyn Stream<Item = Result<MultiTurnStreamItem<R>, StreamingError>> + Send>>;

• MultiTurnStreamItem — exported, users can pattern match on Ok variants
• StreamingError — not exported, users cannot pattern match on Err variants

This inconsistency prevents users from implementing fine-grained error handling or building provider-agnostic abstractions.