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/core/src/codex_thread.rs

@@ -221,9 +221,14 @@ impl CodexThread {
         &self,
         app_server_client_name: Option<String>,
         app_server_client_version: Option<String>,
+        mcp_elicitations_auto_deny: bool,
     ) -> ConstraintResult<()> {
         self.codex
-            .set_app_server_client_info(app_server_client_name, app_server_client_version)
+            .set_app_server_client_info(
+                app_server_client_name,
+                app_server_client_version,
+                mcp_elicitations_auto_deny,
+            )
             .await
     }