MCP server: route structured tool-call requests and expose mcp_protocol [Stack 2/3] (#1751)

- Expose mcp_protocol from mcp-server for reuse in tests and callers.
- In MessageProcessor, detect structured ToolCallRequestParams in
tools/call and forward to a new handler.
- Add handle_new_tool_calls scaffold (returns error for now).
- Test helper: add send_send_user_message_tool_call to McpProcess to
send ConversationSendMessage requests;

This is the second PR in a stack.
Stack:
Final: #1686
Intermediate: #1751
First: #1750

codex-rs/mcp-server/tests/common/mcp_process.rs

@@ -11,8 +11,13 @@ use tokio::process::ChildStdout;
 
 use anyhow::Context;
 use assert_cmd::prelude::*;
+use codex_core::protocol::InputItem;
 use codex_mcp_server::CodexToolCallParam;
 use codex_mcp_server::CodexToolCallReplyParam;
+use codex_mcp_server::mcp_protocol::ConversationId;
+use codex_mcp_server::mcp_protocol::ConversationSendMessageArgs;
+use codex_mcp_server::mcp_protocol::ToolCallRequestParams;
+
 use mcp_types::CallToolRequestParams;
 use mcp_types::ClientCapabilities;
 use mcp_types::Implementation;
@@ -29,6 +34,7 @@ use pretty_assertions::assert_eq;
 use serde_json::json;
 use std::process::Command as StdCommand;
 use tokio::process::Command;
+use uuid::Uuid;
 
 pub struct McpProcess {
     next_request_id: AtomicI64,
@@ -174,6 +180,26 @@ impl McpProcess {
         .await
     }
 
+    pub async fn send_user_message_tool_call(
+        &mut self,
+        message: &str,
+        session_id: &str,
+    ) -> anyhow::Result<i64> {
+        let params = ToolCallRequestParams::ConversationSendMessage(ConversationSendMessageArgs {
+            conversation_id: ConversationId(Uuid::parse_str(session_id)?),
+            content: vec![InputItem::Text {
+                text: message.to_string(),
+            }],
+            parent_message_id: None,
+            conversation_overrides: None,
+        });
+        self.send_request(
+            mcp_types::CallToolRequest::METHOD,
+            Some(serde_json::to_value(params)?),
+        )
+        .await
+    }
+
     async fn send_request(
         &mut self,
         method: &str,