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/core/src/conversation_manager.rs

@@ -0,0 +1,96 @@
+use std::collections::HashMap;
+use std::sync::Arc;
+
+use codex_login::CodexAuth;
+use tokio::sync::RwLock;
+use uuid::Uuid;
+
+use crate::codex::Codex;
+use crate::codex::CodexSpawnOk;
+use crate::codex_conversation::CodexConversation;
+use crate::config::Config;
+use crate::error::CodexErr;
+use crate::error::Result as CodexResult;
+use crate::protocol::Event;
+use crate::protocol::EventMsg;
+use crate::protocol::SessionConfiguredEvent;
+
+/// Represents a newly created Codex conversation, including the first event
+/// (which is [`EventMsg::SessionConfigured`]).
+pub struct NewConversation {
+    pub conversation_id: Uuid,
+    pub conversation: Arc<CodexConversation>,
+    pub session_configured: SessionConfiguredEvent,
+}
+
+/// [`ConversationManager`] is responsible for creating conversations and
+/// maintaining them in memory.
+pub struct ConversationManager {
+    conversations: Arc<RwLock<HashMap<Uuid, Arc<CodexConversation>>>>,
+}
+
+impl Default for ConversationManager {
+    fn default() -> Self {
+        Self {
+            conversations: Arc::new(RwLock::new(HashMap::new())),
+        }
+    }
+}
+
+impl ConversationManager {
+    pub async fn new_conversation(&self, config: Config) -> CodexResult<NewConversation> {
+        let auth = CodexAuth::from_codex_home(&config.codex_home)?;
+        self.new_conversation_with_auth(config, auth).await
+    }
+
+    /// Used for integration tests: should not be used by ordinary business
+    /// logic.
+    pub async fn new_conversation_with_auth(
+        &self,
+        config: Config,
+        auth: Option<CodexAuth>,
+    ) -> CodexResult<NewConversation> {
+        let CodexSpawnOk {
+            codex,
+            init_id,
+            session_id: conversation_id,
+        } = Codex::spawn(config, auth).await?;
+
+        // The first event must be `SessionInitialized`. Validate and forward it
+        // to the caller so that they can display it in the conversation
+        // history.
+        let event = codex.next_event().await?;
+        let session_configured = match event {
+            Event {
+                id,
+                msg: EventMsg::SessionConfigured(session_configured),
+            } if id == init_id => session_configured,
+            _ => {
+                return Err(CodexErr::SessionConfiguredNotFirstEvent);
+            }
+        };
+
+        let conversation = Arc::new(CodexConversation::new(codex));
+        self.conversations
+            .write()
+            .await
+            .insert(conversation_id, conversation.clone());
+
+        Ok(NewConversation {
+            conversation_id,
+            conversation,
+            session_configured,
+        })
+    }
+
+    pub async fn get_conversation(
+        &self,
+        conversation_id: Uuid,
+    ) -> CodexResult<Arc<CodexConversation>> {
+        let conversations = self.conversations.read().await;
+        conversations
+            .get(&conversation_id)
+            .cloned()
+            .ok_or_else(|| CodexErr::ConversationNotFound(conversation_id))
+    }
+}