Introduce a new function to just send user message [Stack 3/3] (#1686)

- MCP server: add send-user-message tool to send user input to a running
Codex session
- Added an integration tests for the happy and sad paths

Changes:
•	Add tool definition and schema.
•	Expose tool in capabilities.
•	Route and handle tool requests with validation.
•	Tests for success, bad UUID, and missing session.


follow‑ups
• Listen path not implemented yet; the tool is present but marked “don’t
use yet” in code comments.
• Session run flag reset: clear running_session_id_set appropriately
after turn completion/errors.

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

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

@@ -20,9 +20,10 @@ mod codex_tool_runner;
 mod exec_approval;
 mod json_to_toml;
 pub mod mcp_protocol;
-mod message_processor;
+pub(crate) mod message_processor;
 mod outgoing_message;
 mod patch_approval;
+pub(crate) mod tool_handlers;
 
 use crate::message_processor::MessageProcessor;
 use crate::outgoing_message::OutgoingMessage;