fix: ensure output of codex-rs/mcp-types/generate_mcp_types.py matches codex-rs/mcp-types/src/lib.rs (#3439)

https://github.com/openai/codex/pull/3395 updated `mcp-types/src/lib.rs`
by hand, but that file is generated code that is produced by
`mcp-types/generate_mcp_types.py`. Unfortunately, we do not have
anything in CI to verify this right now, but I will address that in a
subsequent PR.

#3395 ended up introducing a change that added a required field when
deserializing `InitializeResult`, breaking Codex when used as an MCP
client, so the quick fix in #3436 was to make the new field `Optional`
with `skip_serializing_if = "Option::is_none"`, but that did not address
the problem that `mcp-types/generate_mcp_types.py` and
`mcp-types/src/lib.rs` are out of sync.

This PR gets things back to where they are in sync. It removes the
custom `mcp_types::McpClientInfo` type that was added to
`mcp-types/src/lib.rs` and forces us to use the generated
`mcp_types::Implementation` type. Though this PR also updates
`generate_mcp_types.py` to generate the additional `user_agent:
Optional<String>` field on `Implementation` so that we can continue to
specify it when Codex operates as an MCP server.

However, this also requires us to specify `user_agent: None` when Codex
operates as an MCP client.

We may want to introduce our own `InitializeResult` type that is
specific to when we run as a server to avoid this in the future, but my
immediate goal is just to get things back in sync.

codex-rs/mcp-types/generate_mcp_types.py

@@ -265,8 +265,11 @@ class StructField:
     name: str
     type_name: str
     serde: str | None = None
+    comment: str | None = None
 
     def append(self, out: list[str], supports_const: bool) -> None:
+        if self.comment:
+            out.append(f"    // {self.comment}\n")
         if self.serde:
             out.append(f"    {self.serde}\n")
         if self.viz == "const":
@@ -312,6 +315,18 @@ def define_struct(
         else:
             fields.append(StructField("pub", rs_prop.name, prop_type, rs_prop.serde))
 
+    # Special-case: add Codex-specific user_agent to Implementation
+    if name == "Implementation":
+        fields.append(
+            StructField(
+                "pub",
+                "user_agent",
+                "Option<String>",
+                '#[serde(default, skip_serializing_if = "Option::is_none")]',
+                "This is an extra field that the Codex MCP server sends as part of InitializeResult.",
+            )
+        )
+
     if implements_request_trait(name):
         add_trait_impl(name, "ModelContextProtocolRequest", fields, out)
     elif implements_notification_trait(name):

codex-rs/mcp-types/src/lib.rs

@@ -482,20 +482,12 @@ pub struct ImageContent {
 
 /// Describes the name and version of an MCP implementation, with an optional title for UI representation.
 #[derive(Debug, Clone, PartialEq, Deserialize, Serialize, TS)]
-pub struct McpClientInfo {
-    pub name: String,
-    #[serde(default, skip_serializing_if = "Option::is_none")]
-    pub title: Option<String>,
-    pub version: String,
-}
-
-/// Describes the name and version of an MCP implementation, with an optional title for UI representation.
-#[derive(Debug, Clone, PartialEq, Deserialize, Serialize, TS)]
-pub struct McpServerInfo {
+pub struct Implementation {
     pub name: String,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub title: Option<String>,
     pub version: String,
+    // This is an extra field that the Codex MCP server sends as part of InitializeResult.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub user_agent: Option<String>,
 }
@@ -513,7 +505,7 @@ impl ModelContextProtocolRequest for InitializeRequest {
 pub struct InitializeRequestParams {
     pub capabilities: ClientCapabilities,
     #[serde(rename = "clientInfo")]
-    pub client_info: McpClientInfo,
+    pub client_info: Implementation,
     #[serde(rename = "protocolVersion")]
     pub protocol_version: String,
 }
@@ -527,7 +519,7 @@ pub struct InitializeResult {
     #[serde(rename = "protocolVersion")]
     pub protocol_version: String,
     #[serde(rename = "serverInfo")]
-    pub server_info: McpServerInfo,
+    pub server_info: Implementation,
 }
 
 impl From<InitializeResult> for serde_json::Value {

codex-rs/mcp-types/tests/suite/initialize.rs

@@ -1,10 +1,10 @@
 use mcp_types::ClientCapabilities;
 use mcp_types::ClientRequest;
+use mcp_types::Implementation;
 use mcp_types::InitializeRequestParams;
 use mcp_types::JSONRPC_VERSION;
 use mcp_types::JSONRPCMessage;
 use mcp_types::JSONRPCRequest;
-use mcp_types::McpClientInfo;
 use mcp_types::RequestId;
 use serde_json::json;
 
@@ -58,10 +58,11 @@ fn deserialize_initialize_request() {
                 sampling: None,
                 elicitation: None,
             },
-            client_info: McpClientInfo {
+            client_info: Implementation {
                 name: "acme-client".into(),
                 title: Some("Acme".to_string()),
                 version: "1.2.3".into(),
+                user_agent: None,
             },
             protocol_version: "2025-06-18".into(),
         }