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/Cargo.lock

@@ -2643,6 +2643,7 @@ version = "0.0.0"
 dependencies = [
  "anyhow",
  "assert_cmd",
+ "codex-core",
  "codex-mcp-server",
  "mcp-types",
  "pretty_assertions",
@@ -2650,6 +2651,7 @@ dependencies = [
  "shlex",
  "tempfile",
  "tokio",
+ "uuid",
  "wiremock",
 ]