chore: introduce ConversationManager as a clearinghouse for all conversations (#2240)

This PR does two things because after I got deep into the first one I
started pulling on the thread to the second:

- Makes `ConversationManager` the place where all in-memory
conversations are created and stored. Previously, `MessageProcessor` in
the `codex-mcp-server` crate was doing this via its `session_map`, but
this is something that should be done in `codex-core`.
- It unwinds the `ctrl_c: tokio::sync::Notify` that was threaded
throughout our code. I think this made sense at one time, but now that
we handle Ctrl-C within the TUI and have a proper `Op::Interrupt` event,
I don't think this was quite right, so I removed it. For `codex exec`
and `codex proto`, we now use `tokio::signal::ctrl_c()` directly, but we
no longer make `Notify` a field of `Codex` or `CodexConversation`.

Changes of note:

- Adds the files `conversation_manager.rs` and `codex_conversation.rs`
to `codex-core`.
- `Codex` and `CodexSpawnOk` are no longer exported from `codex-core`:
other crates must use `CodexConversation` instead (which is created via
`ConversationManager`).
- `core/src/codex_wrapper.rs` has been deleted in favor of
`ConversationManager`.
- `ConversationManager::new_conversation()` returns `NewConversation`,
which is in line with the `new_conversation` tool we want to add to the
MCP server. Note `NewConversation` includes `SessionConfiguredEvent`, so
we eliminate checks in cases like `codex-rs/core/tests/client.rs` to
verify `SessionConfiguredEvent` is the first event because that is now
internal to `ConversationManager`.
- Quite a bit of code was deleted from
`codex-rs/mcp-server/src/message_processor.rs` since it no longer has to
manage multiple conversations itself: it goes through
`ConversationManager` instead.
- `core/tests/live_agent.rs` has been deleted because I had to update a
bunch of tests and all the tests in here were ignored, and I don't think
anyone ever ran them, so this was just technical debt, at this point.
- Removed `notify_on_sigint()` from `util.rs` (and in a follow-up, I
hope to refactor the blandly-named `util.rs` into more descriptive
files).
- In general, I started replacing local variables named `codex` as
`conversation`, where appropriate, though admittedly I didn't do it
through all the integration tests because that would have added a lot of
noise to this PR.




---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/2240).
* #2264
* #2263
* __->__ #2240

codex-rs/tui/src/chatwidget/agent.rs

@@ -1,7 +1,7 @@
 use std::sync::Arc;
 
-use codex_core::codex_wrapper::CodexConversation;
-use codex_core::codex_wrapper::init_codex;
+use codex_core::ConversationManager;
+use codex_core::NewConversation;
 use codex_core::config::Config;
 use codex_core::protocol::Op;
 use tokio::sync::mpsc::UnboundedSender;
@@ -12,17 +12,21 @@ use crate::app_event_sender::AppEventSender;
 
 /// Spawn the agent bootstrapper and op forwarding loop, returning the
 /// `UnboundedSender<Op>` used by the UI to submit operations.
-pub(crate) fn spawn_agent(config: Config, app_event_tx: AppEventSender) -> UnboundedSender<Op> {
+pub(crate) fn spawn_agent(
+    config: Config,
+    app_event_tx: AppEventSender,
+    server: Arc<ConversationManager>,
+) -> UnboundedSender<Op> {
     let (codex_op_tx, mut codex_op_rx) = unbounded_channel::<Op>();
 
     let app_event_tx_clone = app_event_tx.clone();
     tokio::spawn(async move {
-        let CodexConversation {
-            codex,
+        let NewConversation {
+            conversation_id: _,
+            conversation,
             session_configured,
-            ..
-        } = match init_codex(config).await {
-            Ok(vals) => vals,
+        } = match server.new_conversation(config).await {
+            Ok(v) => v,
             Err(e) => {
                 // TODO: surface this error to the user.
                 tracing::error!("failed to initialize codex: {e}");
@@ -30,21 +34,25 @@ pub(crate) fn spawn_agent(config: Config, app_event_tx: AppEventSender) -> Unbou
             }
         };
 
-        // Forward the captured `SessionInitialized` event that was consumed
-        // inside `init_codex()` so it can be rendered in the UI.
-        app_event_tx_clone.send(AppEvent::CodexEvent(session_configured.clone()));
-        let codex = Arc::new(codex);
-        let codex_clone = codex.clone();
+        // Forward the captured `SessionConfigured` event so it can be rendered in the UI.
+        let ev = codex_core::protocol::Event {
+            // The `id` does not matter for rendering, so we can use a fake value.
+            id: "".to_string(),
+            msg: codex_core::protocol::EventMsg::SessionConfigured(session_configured),
+        };
+        app_event_tx_clone.send(AppEvent::CodexEvent(ev));
+
+        let conversation_clone = conversation.clone();
         tokio::spawn(async move {
             while let Some(op) = codex_op_rx.recv().await {
-                let id = codex_clone.submit(op).await;
+                let id = conversation_clone.submit(op).await;
                 if let Err(e) = id {
                     tracing::error!("failed to submit op: {e}");
                 }
             }
         });
 
-        while let Ok(event) = codex.next_event().await {
+        while let Ok(event) = conversation.next_event().await {
             app_event_tx_clone.send(AppEvent::CodexEvent(event));
         }
     });

codex-rs/tui/src/chatwidget/tests.rs

@@ -104,7 +104,8 @@ async fn helpers_are_available_and_do_not_panic() {
     let (tx_raw, _rx) = channel::<AppEvent>();
     let tx = AppEventSender::new(tx_raw);
     let cfg = test_config();
-    let mut w = ChatWidget::new(cfg, tx, None, Vec::new(), false);
+    let conversation_manager = Arc::new(ConversationManager::default());
+    let mut w = ChatWidget::new(cfg, conversation_manager, tx, None, Vec::new(), false);
     // Basic construction sanity.
     let _ = &mut w;
 }