MCP: add conversation.create tool [Stack 2/2] (#1783)

Introduce conversation.create handler (handle_create_conversation) and
wire it in MessageProcessor.

Stack:
Top: #1783 
Bottom: #1784

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Co-authored-by: Gabriel Peal <gpeal@users.noreply.github.com>

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

@@ -17,6 +17,7 @@ use tracing_subscriber::EnvFilter;
 
 mod codex_tool_config;
 mod codex_tool_runner;
+mod conversation_loop;
 mod exec_approval;
 mod json_to_toml;
 pub mod mcp_protocol;