[plugins] Support plugin installation elicitation. (#14896)

It now supports:

- Connectors that are from installed and enabled plugins that are not
installed yet
- Plugins that are on the allowlist that are not installed yet.

codex-rs/tui/src/bottom_pane/mod.rs

@@ -965,7 +965,9 @@ impl BottomPane {
             request
         };
 
-        if let Some(tool_suggestion) = request.tool_suggestion() {
+        if let Some(tool_suggestion) = request.tool_suggestion()
+            && let Some(install_url) = tool_suggestion.install_url.clone()
+        {
             let suggestion_type = match tool_suggestion.suggest_type {
                 mcp_server_elicitation::ToolSuggestionType::Install => {
                     AppLinkSuggestionType::Install
@@ -989,7 +991,7 @@ impl BottomPane {
                             "Enable this app to use it for the current request.".to_string()
                         }
                     },
-                    url: tool_suggestion.install_url.clone(),
+                    url: install_url,
                     is_installed,
                     is_enabled: false,
                     suggest_reason: Some(tool_suggestion.suggest_reason.clone()),