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/tool_handlers/mod.rs

@@ -1 +1,2 @@
+pub(crate) mod create_conversation;
 pub(crate) mod send_message;