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/app-server/src/message_processor.rs

@@ -954,12 +954,22 @@ impl MessageProcessor {
             }
             ClientRequest::ThreadResume { params, .. } => {
                 self.thread_processor
-                    .thread_resume(request_id.clone(), params)
+                    .thread_resume(
+                        request_id.clone(),
+                        params,
+                        app_server_client_name.clone(),
+                        client_version.clone(),
+                    )
                     .await
             }
             ClientRequest::ThreadFork { params, .. } => {
                 self.thread_processor
-                    .thread_fork(request_id.clone(), params)
+                    .thread_fork(
+                        request_id.clone(),
+                        params,
+                        app_server_client_name.clone(),
+                        client_version.clone(),
+                    )
                     .await
             }
             ClientRequest::ThreadArchive { params, .. } => {