feat(core): Add remove_conversation to ConversationManager for ma… (#2613)

### What this PR does

This PR introduces a new public method,
remove_conversation(conversation_id: Uuid), to the ConversationManager.
This allows consumers of the codex-core library to manually remove a
conversation from the manager's in-memory storage.

### Why this change is needed
I am currently adapting the Codex client to run as a long-lived server
application. In this server environment, ConversationManager instances
persist for extended periods, and new conversations are created for each
incoming user request.

The current implementation of ConversationManager stores all created
conversations in a HashMap indefinitely, with no mechanism for removal.
This leads to unbounded memory growth in a server context, as every new
conversation permanently occupies memory.

While an automatic TTL-based cleanup mechanism could be one solution, a
simpler, more direct remove_conversation method provides the necessary
control for my use case. It allows my server application to explicitly
manage the lifecycle of conversations, such as cleaning them up after a
request is fully processed or after a period of inactivity is detected
at the application level.

This change provides a minimal, non-intrusive way to address the memory
management issue for server-like applications built on top of
codex-core, giving developers the flexibility to implement their own
cleanup logic.

Signed-off-by: M4n5ter <m4n5terrr@gmail.com>
Co-authored-by: Michael Bolin <mbolin@openai.com>

codex-rs/core/src/conversation_manager.rs

@@ -110,6 +110,10 @@ impl ConversationManager {
             .ok_or_else(|| CodexErr::ConversationNotFound(conversation_id))
     }
 
+    pub async fn remove_conversation(&self, conversation_id: Uuid) {
+        self.conversations.write().await.remove(&conversation_id);
+    }
+
     /// Fork an existing conversation by dropping the last `drop_last_messages`
     /// user/assistant messages from its transcript and starting a new
     /// conversation with identical configuration (unless overridden by the