feat: Reify SSE state machine (#1428)

* feat: harden SSE state machine

* feat: flatten GenericEventSource

* Unbox retry strategy

* doc comments

Author: FayCarsons

rig/rig-core/examples/anthropic_streaming.rs

@@ -8,7 +8,7 @@ use rig::{
 async fn main() -> Result<(), anyhow::Error> {
     // Create streaming agent with a single context prompt
     let agent = anthropic::Client::from_env()
-        .agent(anthropic::completion::CLAUDE_3_5_SONNET)
+        .agent(anthropic::completion::CLAUDE_4_SONNET)
         .preamble("Be precise and concise.")
         .temperature(0.5)
         .build();

rig/rig-core/examples/anthropic_streaming_with_tools.rs

@@ -89,7 +89,7 @@ async fn main() -> Result<(), anyhow::Error> {
     tracing_subscriber::fmt().init();
     // Create agent with a single context prompt and two tools
     let calculator_agent = providers::anthropic::Client::from_env()
-        .agent(providers::anthropic::completion::CLAUDE_3_5_SONNET)
+        .agent(providers::anthropic::completion::CLAUDE_4_SONNET)
         .preamble(
             "You are a calculator here to help the user perform arithmetic
             operations. Use the tools provided to answer the user's question.