fix: remove mcp-types from app server protocol (#4537)

We continue the separation between `codex app-server` and `codex
mcp-server`.

In particular, we introduce a new crate, `codex-app-server-protocol`,
and migrate `codex-rs/protocol/src/mcp_protocol.rs` into it, renaming it
`codex-rs/app-server-protocol/src/protocol.rs`.

Because `ConversationId` was defined in `mcp_protocol.rs`, we move it
into its own file, `codex-rs/protocol/src/conversation_id.rs`, and
because it is referenced in a ton of places, we have to touch a lot of
files as part of this PR.

We also decide to get away from proper JSON-RPC 2.0 semantics, so we
also introduce `codex-rs/app-server-protocol/src/jsonrpc_lite.rs`, which
is basically the same `JSONRPCMessage` type defined in `mcp-types`
except with all of the `"jsonrpc": "2.0"` removed.

Getting rid of `"jsonrpc": "2.0"` makes our serialization logic
considerably simpler, as we can lean heavier on serde to serialize
directly into the wire format that we use now.

codex-rs/protocol/src/conversation_id.rs

@@ -0,0 +1,68 @@
+use std::fmt::Display;
+
+use serde::Deserialize;
+use serde::Serialize;
+use ts_rs::TS;
+use uuid::Uuid;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, TS, Hash)]
+#[ts(type = "string")]
+pub struct ConversationId {
+    uuid: Uuid,
+}
+
+impl ConversationId {
+    pub fn new() -> Self {
+        Self {
+            uuid: Uuid::now_v7(),
+        }
+    }
+
+    pub fn from_string(s: &str) -> Result<Self, uuid::Error> {
+        Ok(Self {
+            uuid: Uuid::parse_str(s)?,
+        })
+    }
+}
+
+impl Default for ConversationId {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl Display for ConversationId {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.uuid)
+    }
+}
+
+impl Serialize for ConversationId {
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
+    where
+        S: serde::Serializer,
+    {
+        serializer.collect_str(&self.uuid)
+    }
+}
+
+impl<'de> Deserialize<'de> for ConversationId {
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        let value = String::deserialize(deserializer)?;
+        let uuid = Uuid::parse_str(&value).map_err(serde::de::Error::custom)?;
+        Ok(Self { uuid })
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn test_conversation_id_default_is_not_zeroes() {
+        let id = ConversationId::default();
+        assert_ne!(id.uuid, Uuid::nil());
+    }
+}

codex-rs/protocol/src/lib.rs

@@ -1,6 +1,7 @@
+mod conversation_id;
+pub use conversation_id::ConversationId;
 pub mod config_types;
 pub mod custom_prompts;
-pub mod mcp_protocol;
 pub mod message_history;
 pub mod models;
 pub mod num_format;

codex-rs/protocol/src/protocol.rs

@@ -10,10 +10,10 @@ use std::path::PathBuf;
 use std::str::FromStr;
 use std::time::Duration;
 
+use crate::ConversationId;
 use crate::config_types::ReasoningEffort as ReasoningEffortConfig;
 use crate::config_types::ReasoningSummary as ReasoningSummaryConfig;
 use crate::custom_prompts::CustomPrompt;
-use crate::mcp_protocol::ConversationId;
 use crate::message_history::HistoryEntry;
 use crate::models::ContentItem;
 use crate::models::ResponseItem;