TUI: route elicitation responses to request thread (#23241)

## Why

Fixes #21894.

When the TUI handles an MCP elicitation, the request payload already
includes the thread that generated the elicitation.
`ChatWidget::handle_elicitation_request_now` was ignoring that value and
using the currently visible chat thread instead. In a multi-session TUI,
that can send `resolve_elicitation` to an older visible thread rather
than the session that owns the pending elicitation, producing
`elicitation request not found` and leaving the prompt unresolved.

## What changed

- Parse `McpServerElicitationRequestParams.thread_id` in the ChatWidget
elicitation handler and use it for app-link, form, fallback approval,
and auto-decline resolution paths.
- Keep the existing visible-thread fallback only for malformed request
payloads with an invalid thread id.
- Update the invalid URL elicitation regression test so the visible
thread and request thread intentionally differ.

codex-rs/tui/src/chatwidget/tests/app_server.rs

@@ -4,13 +4,14 @@ use pretty_assertions::assert_eq;
 #[tokio::test]
 async fn invalid_url_elicitation_is_declined() {
     let (mut chat, _app_event_tx, mut rx, _op_rx) = make_chatwidget_manual_with_sender().await;
-    let thread_id = ThreadId::new();
+    let visible_thread_id = ThreadId::new();
-    chat.thread_id = Some(thread_id);
+    let request_thread_id = ThreadId::new();
+    chat.thread_id = Some(visible_thread_id);
 
     chat.handle_elicitation_request_now(
         codex_app_server_protocol::RequestId::Integer(9),
         codex_app_server_protocol::McpServerElicitationRequestParams {
-            thread_id: thread_id.to_string(),
+            thread_id: request_thread_id.to_string(),
             turn_id: Some("turn-auth".to_string()),
             server_name: "payments".to_string(),
             request: codex_app_server_protocol::McpServerElicitationRequest::Url {
@@ -33,7 +34,7 @@ async fn invalid_url_elicitation_is_declined() {
                 content: None,
                 meta: None,
             },
-        }) if op_thread_id == thread_id && server_name == "payments"
+        }) if op_thread_id == request_thread_id && server_name == "payments"
     );
 }
 

codex-rs/tui/src/chatwidget/tool_requests.rs

@@ -341,7 +341,8 @@ impl ChatWidget {
             server_name: params.server_name.clone(),
         });
 
-        let thread_id = self.thread_id.unwrap_or_default();
+        let thread_id = ThreadId::from_string(&params.thread_id)
+            .unwrap_or_else(|_| self.thread_id.unwrap_or_default());
         if let Some(params) = crate::bottom_pane::AppLinkViewParams::from_url_app_server_request(
             thread_id,
             &params.server_name,