Everything is streaming (#7247)

Co-authored-by: Douwe Osinga <douwe@squareup.com>
Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

crates/goose/tests/mcp_integration_test.rs

@@ -19,7 +19,10 @@ use test_case::test_case;
 
 use async_trait::async_trait;
 use goose::conversation::message::Message;
-use goose::providers::base::{Provider, ProviderDef, ProviderMetadata, ProviderUsage, Usage};
+use goose::providers::base::{
+    stream_from_single_message, MessageStream, Provider, ProviderDef, ProviderMetadata,
+    ProviderUsage, Usage,
+};
 use goose::providers::errors::ProviderError;
 use once_cell::sync::Lazy;
 use std::process::Command;
@@ -69,18 +72,17 @@ impl Provider for MockProvider {
         "mock"
     }
 
-    async fn complete_with_model(
+    async fn stream(
         &self,
-        _session_id: Option<&str>,
         _model_config: &ModelConfig,
+        _session_id: &str,
         _system: &str,
         _messages: &[Message],
         _tools: &[Tool],
-    ) -> anyhow::Result<(Message, ProviderUsage), ProviderError> {
-        Ok((
-            Message::assistant().with_text("\"So we beat on, boats against the current, borne back ceaselessly into the past.\" — F. Scott Fitzgerald, The Great Gatsby (1925)"),
-            ProviderUsage::new("mock".to_string(), Usage::default()),
-        ))
+    ) -> Result<MessageStream, ProviderError> {
+        let message = Message::assistant().with_text("\"So we beat on, boats against the current, borne back ceaselessly into the past.\" — F. Scott Fitzgerald, The Great Gatsby (1925)");
+        let usage = ProviderUsage::new("mock".to_string(), Usage::default());
+        Ok(stream_from_single_message(message, usage))
     }
 
     fn get_model_config(&self) -> ModelConfig {