Finish moving codex exec to app-server (#15424)

This PR completes the conversion of non-interactive `codex exec` to use
app server rather than directly using core events and methods.

### Summary
- move `codex-exec` off exec-owned `AuthManager` and `ThreadManager`
state
- route exec bootstrap, resume, and auth refresh through existing
app-server paths
- replace legacy `codex/event/*` decoding in exec with typed app-server
notification handling
- update human and JSONL exec output adapters to translate existing
app-server notifications only
- clean up "app server client" layer by eliminating support for legacy
notifications; this is no longer needed
- remove exposure of `authManager` and `threadManager` from "app server
client" layer

### Testing
- `exec` has pretty extensive unit and integration tests already, and
these all pass
- In addition, I asked Codex to put together a comprehensive manual set
of tests to cover all of the `codex exec` functionality (including
command-line options), and it successfully generated and ran these tests

codex-rs/exec/src/event_processor.rs

@@ -1,13 +1,13 @@
 use std::path::Path;
 
+use codex_app_server_protocol::ServerNotification;
 use codex_core::config::Config;
-use codex_protocol::protocol::Event;
 use codex_protocol::protocol::SessionConfiguredEvent;
 
-pub(crate) enum CodexStatus {
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum CodexStatus {
     Running,
     InitiateShutdown,
-    Shutdown,
 }
 
 pub(crate) trait EventProcessor {
@@ -19,8 +19,11 @@ pub(crate) trait EventProcessor {
         session_configured: &SessionConfiguredEvent,
     );
 
-    /// Handle a single event emitted by the agent.
-    fn process_event(&mut self, event: Event) -> CodexStatus;
+    /// Handle a single typed app-server notification emitted by the agent.
+    fn process_server_notification(&mut self, notification: ServerNotification) -> CodexStatus;
+
+    /// Handle a local exec warning that is not represented as an app-server notification.
+    fn process_warning(&mut self, message: String) -> CodexStatus;
 
     fn print_final_output(&mut self) {}
 }