Auto-deny MCP elicitations for Xcode 26.4 clients (#21113)

## Summary

Xcode 26.4 was built against app-server behavior from before MCP
elicitation requests became client-visible in CLI 0.120.0 via #17043.
That client line does not expect the new events/messages, so this PR
restores the old behavior for exactly that client/version combination.

The compatibility handling stays in the app-server layer: when the
initialized client is `Xcode` and its version starts with `26.4`, the
app server marks the live Codex thread so MCP elicitations are
auto-denied. The flag is applied on thread start/resume/fork/turn
attachment, carried through `Codex`/`CodexThread`, and stored on
`McpConnectionManager` so refreshed MCP managers preserve the behavior.

## Notes

This is intentionally narrow and includes a TODO to remove the
compatibility path once Xcode 26.4 ages out.

codex-rs/codex-mcp/src/connection_manager.rs

@@ -128,6 +128,14 @@ impl McpConnectionManager {
         }
     }
 
+    pub fn elicitations_auto_deny(&self) -> bool {
+        self.elicitation_requests.auto_deny()
+    }
+
+    pub fn set_elicitations_auto_deny(&self, auto_deny: bool) {
+        self.elicitation_requests.set_auto_deny(auto_deny);
+    }
+
     #[allow(clippy::new_ret_no_self, clippy::too_many_arguments)]
     pub async fn new(
         mcp_servers: &HashMap<String, McpServerConfig>,