Add notifier tests (#4064)

Proposal:
1. Use anyhow for tests and avoid unwrap
2. Extract a helper for starting a test instance of codex

codex-rs/core/tests/common/test_codex.rs

@@ -0,0 +1,75 @@
+use std::mem::swap;
+use std::sync::Arc;
+
+use codex_core::CodexAuth;
+use codex_core::CodexConversation;
+use codex_core::ConversationManager;
+use codex_core::ModelProviderInfo;
+use codex_core::NewConversation;
+use codex_core::built_in_model_providers;
+use codex_core::config::Config;
+use codex_core::protocol::SessionConfiguredEvent;
+use tempfile::TempDir;
+
+use crate::load_default_config_for_test;
+
+type ConfigMutator = dyn FnOnce(&mut Config);
+
+pub struct TestCodexBuilder {
+    config_mutators: Vec<Box<ConfigMutator>>,
+}
+
+impl TestCodexBuilder {
+    pub fn with_config<T>(mut self, mutator: T) -> Self
+    where
+        T: FnOnce(&mut Config) + 'static,
+    {
+        self.config_mutators.push(Box::new(mutator));
+        self
+    }
+
+    pub async fn build(&mut self, server: &wiremock::MockServer) -> anyhow::Result<TestCodex> {
+        // Build config pointing to the mock server and spawn Codex.
+        let model_provider = ModelProviderInfo {
+            base_url: Some(format!("{}/v1", server.uri())),
+            ..built_in_model_providers()["openai"].clone()
+        };
+        let home = TempDir::new()?;
+        let cwd = TempDir::new()?;
+        let mut config = load_default_config_for_test(&home);
+        config.cwd = cwd.path().to_path_buf();
+        config.model_provider = model_provider;
+        let mut mutators = vec![];
+        swap(&mut self.config_mutators, &mut mutators);
+
+        for mutator in mutators {
+            mutator(&mut config)
+        }
+        let conversation_manager = ConversationManager::with_auth(CodexAuth::from_api_key("dummy"));
+        let NewConversation {
+            conversation,
+            session_configured,
+            ..
+        } = conversation_manager.new_conversation(config).await?;
+
+        Ok(TestCodex {
+            home,
+            cwd,
+            codex: conversation,
+            session_configured,
+        })
+    }
+}
+
+pub struct TestCodex {
+    pub home: TempDir,
+    pub cwd: TempDir,
+    pub codex: Arc<CodexConversation>,
+    pub session_configured: SessionConfiguredEvent,
+}
+
+pub fn test_codex() -> TestCodexBuilder {
+    TestCodexBuilder {
+        config_mutators: vec![],
+    }
+}