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/tool_handlers/mod.rs

@@ -0,0 +1 @@
+pub(crate) mod send_message;