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/Cargo.toml

@@ -10,6 +10,7 @@ path = "lib.rs"
 anyhow = "1"
 assert_cmd = "2"
 codex-mcp-server = { path = "../.." }
+codex-core = { path = "../../../core" }
 mcp-types = { path = "../../../mcp-types" }
 pretty_assertions = "1.4.1"
 serde_json = "1"
@@ -22,3 +23,4 @@ tokio = { version = "1", features = [
     "rt-multi-thread",
 ] }
 wiremock = "0.6"
+uuid = { version = "1", features = ["serde", "v4"] }