Move TUI on top of app server (parallel code) (#14717)

This PR replicates the `tui` code directory and creates a temporary
parallel `tui_app_server` directory. It also implements a new feature
flag `tui_app_server` to select between the two tui implementations.

Once the new app-server-based TUI is stabilized, we'll delete the old
`tui` directory and feature flag.

codex-rs/tui_app_server/tests/manager_dependency_regression.rs

@@ -0,0 +1,52 @@
+use std::fs;
+use std::path::Path;
+use std::path::PathBuf;
+
+fn rust_sources_under(dir: &Path) -> Vec<PathBuf> {
+    let mut files = Vec::new();
+    let entries =
+        fs::read_dir(dir).unwrap_or_else(|err| panic!("failed to read {}: {err}", dir.display()));
+    for entry in entries {
+        let entry = entry.unwrap_or_else(|err| panic!("failed to read dir entry: {err}"));
+        let path = entry.path();
+        if path.is_dir() {
+            files.extend(rust_sources_under(&path));
+        } else if path.extension().is_some_and(|ext| ext == "rs") {
+            files.push(path);
+        }
+    }
+    files.sort();
+    files
+}
+
+#[test]
+fn tui_app_server_runtime_source_does_not_depend_on_manager_escape_hatches() {
+    let src_dir = codex_utils_cargo_bin::find_resource!("src")
+        .unwrap_or_else(|err| panic!("failed to resolve src runfile: {err}"));
+    let sources = rust_sources_under(&src_dir);
+    let forbidden = [
+        "AuthManager",
+        "ThreadManager",
+        "auth_manager(",
+        "thread_manager(",
+    ];
+
+    let violations: Vec<String> = sources
+        .iter()
+        .flat_map(|path| {
+            let contents = fs::read_to_string(path)
+                .unwrap_or_else(|err| panic!("failed to read {}: {err}", path.display()));
+            let path_display = path.display().to_string();
+            forbidden
+                .iter()
+                .filter(move |needle| contents.contains(**needle))
+                .map(move |needle| format!("{path_display} contains `{needle}`"))
+        })
+        .collect();
+
+    assert!(
+        violations.is_empty(),
+        "unexpected manager dependency regression(s):\n{}",
+        violations.join("\n")
+    );
+}