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/src/lib.rs

@@ -19,7 +19,7 @@ mod codex_tool_config;
 mod codex_tool_runner;
 mod exec_approval;
 mod json_to_toml;
-mod mcp_protocol;
+pub mod mcp_protocol;
 mod message_processor;
 mod outgoing_message;
 mod patch_approval;