Initial plugins TUI menu - list and read only. tui + tui_app_server (#15215)

### Preliminary /plugins TUI menu
- Adds a preliminary /plugins menu flow in both tui and tui_app_server.
- Fetches plugin list data asynchronously and shows loading/error/cached
states.
  - Limits this first pass to the curated ChatGPT marketplace.
  - Shows available plugins with installed/status metadata.
- Supports in-menu search over plugin display name, plugin id, plugin
name, and marketplace label.
- Opens a plugin detail view on selection, including summaries for
Skills, Apps, and MCP Servers, with back navigation.

### Testing
  - Launch codex-cli with plugins enabled (`--enable plugins`).
  - Run /plugins and verify:
      - loading state appears first
      - plugin list is shown
      - search filters results
- selecting a plugin opens detail view, with a list of
skills/connectors/MCP servers for the plugin
      - back action returns to the list.
- Verify disabled behavior by running /plugins without plugins enabled
(shows “Plugins are disabled” message).
- Launch with `--enable tui_app_server` (and plugins enabled) and repeat
the same /plugins flow; behavior should match.

codex-rs/tui/src/slash_command.rs

@@ -43,6 +43,7 @@ pub enum SlashCommand {
     Theme,
     Mcp,
     Apps,
+    Plugins,
     Logout,
     Quit,
     Exit,
@@ -110,6 +111,7 @@ impl SlashCommand {
             SlashCommand::Experimental => "toggle experimental features",
             SlashCommand::Mcp => "list configured MCP tools",
             SlashCommand::Apps => "manage apps",
+            SlashCommand::Plugins => "browse plugins",
             SlashCommand::Logout => "log out of Codex",
             SlashCommand::Rollout => "print the rollout file path",
             SlashCommand::TestApproval => "test approval request",
@@ -168,6 +170,7 @@ impl SlashCommand {
             | SlashCommand::Stop
             | SlashCommand::Mcp
             | SlashCommand::Apps
+            | SlashCommand::Plugins
             | SlashCommand::Feedback
             | SlashCommand::Quit
             | SlashCommand::Exit => true,