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)
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with [ReviewStack](https://reviewstack.dev/openai/codex/pull/2240).
* #2264
* #2263
* __->__ #2240

codex-rs/mcp-server/src/tool_handlers/send_message.rs

@@ -1,12 +1,6 @@
-use std::collections::HashMap;
-use std::sync::Arc;
-
-use codex_core::Codex;
 use codex_core::protocol::Op;
 use codex_core::protocol::Submission;
 use mcp_types::RequestId;
-use tokio::sync::Mutex;
-use uuid::Uuid;
 
 use crate::mcp_protocol::ConversationSendMessageArgs;
 use crate::mcp_protocol::ConversationSendMessageResult;
@@ -41,7 +35,11 @@ pub(crate) async fn handle_send_message(
     }
 
     let session_id = conversation_id.0;
-    let Some(codex) = get_session(session_id, message_processor.session_map()).await else {
+    let Ok(codex) = message_processor
+        .get_conversation_manager()
+        .get_conversation(session_id)
+        .await
+    else {
         message_processor
             .send_response_with_optional_error(
                 id,
@@ -114,11 +112,3 @@ pub(crate) async fn handle_send_message(
         )
         .await;
 }
-
-pub(crate) async fn get_session(
-    session_id: Uuid,
-    session_map: Arc<Mutex<HashMap<Uuid, Arc<Codex>>>>,
-) -> Option<Arc<Codex>> {
-    let guard = session_map.lock().await;
-    guard.get(&session_id).cloned()
-}