test: isolate app-server-client in-process test state (#21328)

## Why

The in-process `app-server-client` tests were still building their
configs from the ambient `codex_home` and letting the embedded app
server create its own state DB when `state_db` was absent. That matters
because in-process startup falls back to
`init_state_db_from_config(...)` in that case, so tests can otherwise
share persisted state instead of getting isolated fixtures:
[`app-server/src/in_process.rs`](a98623511b/codex-rs/app-server/src/in_process.rs (L368-L373)).

## What changed

- Give each in-process test client its own temporary `codex_home`.
- Initialize the matching state DB from that per-client config and pass
it into the client explicitly.
- Keep the temp directory alive for the lifetime of the test client
through a small `TestClient` wrapper.
- Add `tempfile` as a dev dependency for the new harness.

The updated setup lives in
[`app-server-client/src/lib.rs`](35c1133d45/codex-rs/app-server-client/src/lib.rs (L982-L1055)).

## Testing

- Existing `codex-app-server-client` tests continue to exercise the
updated in-process client path through the isolated helper.

codex-rs/Cargo.lock

@@ -1944,6 +1944,7 @@ dependencies = [
  "pretty_assertions",
  "serde",
  "serde_json",
+ "tempfile",
  "tokio",
  "tokio-tungstenite",
  "toml 0.9.11+spec-1.1.0",

codex-rs/app-server-client/Cargo.toml

@@ -33,4 +33,5 @@ url = { workspace = true }
 [dev-dependencies]
 pretty_assertions = { workspace = true }
 serde_json = { workspace = true }
+tempfile = { workspace = true }
 tokio = { workspace = true, features = ["macros", "rt-multi-thread"] }

codex-rs/app-server-client/src/lib.rs

@@ -954,9 +954,13 @@ mod tests {
     use codex_app_server_protocol::ToolRequestUserInputParams;
     use codex_app_server_protocol::ToolRequestUserInputQuestion;
     use codex_core::config::ConfigBuilder;
+    use codex_core::init_state_db_from_config;
     use futures::SinkExt;
     use futures::StreamExt;
     use pretty_assertions::assert_eq;
+    use std::ops::Deref;
+    use std::path::Path;
+    use tempfile::TempDir;
     use tokio::net::TcpListener;
     use tokio::time::Duration;
     use tokio::time::timeout;
@@ -975,19 +979,59 @@ mod tests {
         }
     }
 
+    async fn build_test_config_for_codex_home(codex_home: &Path) -> Config {
+        match ConfigBuilder::default()
+            .codex_home(codex_home.to_path_buf())
+            .build()
+            .await
+        {
+            Ok(config) => config,
+            Err(_) => Config::load_default_with_cli_overrides_for_codex_home(
+                codex_home.to_path_buf(),
+                Vec::new(),
+            )
+            .await
+            .expect("default config should load"),
+        }
+    }
+
+    struct TestClient {
+        _codex_home: TempDir,
+        client: InProcessAppServerClient,
+    }
+
+    impl Deref for TestClient {
+        type Target = InProcessAppServerClient;
+
+        fn deref(&self) -> &Self::Target {
+            &self.client
+        }
+    }
+
+    impl TestClient {
+        async fn shutdown(self) -> IoResult<()> {
+            self.client.shutdown().await
+        }
+    }
+
     async fn start_test_client_with_capacity(
         session_source: SessionSource,
         channel_capacity: usize,
-    ) -> InProcessAppServerClient {
-        InProcessAppServerClient::start(InProcessClientStartArgs {
+    ) -> TestClient {
+        let codex_home = TempDir::new().expect("temp dir");
+        let config = Arc::new(build_test_config_for_codex_home(codex_home.path()).await);
+        let state_db = init_state_db_from_config(config.as_ref())
+            .await
+            .expect("state db should initialize for in-process test");
+        let client = InProcessAppServerClient::start(InProcessClientStartArgs {
             arg0_paths: Arg0DispatchPaths::default(),
-            config: Arc::new(build_test_config().await),
+            config,
             cli_overrides: Vec::new(),
             loader_overrides: LoaderOverrides::default(),
             cloud_requirements: CloudRequirementsLoader::default(),
             feedback: CodexFeedback::new(),
             log_db: None,
-            state_db: None,
+            state_db: Some(state_db),
             environment_manager: Arc::new(EnvironmentManager::default_for_tests()),
             config_warnings: Vec::new(),
             session_source,
@@ -999,10 +1043,15 @@ mod tests {
             channel_capacity,
         })
         .await
-        .expect("in-process app-server client should start")
+        .expect("in-process app-server client should start");
+
+        TestClient {
+            _codex_home: codex_home,
+            client,
+        }
     }
 
-    async fn start_test_client(session_source: SessionSource) -> InProcessAppServerClient {
+    async fn start_test_client(session_source: SessionSource) -> TestClient {
         start_test_client_with_capacity(session_source, DEFAULT_IN_PROCESS_CHANNEL_CAPACITY).await
     }