[codex] [4/4] Simplify recommended plugin install schema (#28403)

## Summary
- Simplify recommendation-context `request_plugin_install` arguments to
`plugin_id` and `suggest_reason`.
- Derive plugin type and install action from the matched candidate while
preserving Codex-owned elicitation metadata.
- Keep the legacy list-backed schema unchanged and accept resumed calls
that still use `tool_id`.

## Stack
- #28399
- #28400
- #27704
- This PR

## Validation
- `just test -p codex-tools -p codex-core request_plugin_install` (25
passed)
- `just fix -p codex-tools -p codex-core`
- `just fmt`
- `git diff --check`

codex-rs/tools/src/request_plugin_install_tests.rs

@@ -5,12 +5,6 @@ use serde_json::json;
 
 #[test]
 fn build_request_plugin_install_elicitation_request_uses_expected_shape() {
-    let args = RequestPluginInstallArgs {
-        tool_type: DiscoverableToolType::Connector,
-        action_type: DiscoverableToolAction::Install,
-        tool_id: "connector_2128aebfecb84f64a069897515042a44".to_string(),
-        suggest_reason: "Plan and reference events from your calendar".to_string(),
-    };
     let connector = DiscoverableTool::Connector(Box::new(AppInfo {
         id: "connector_2128aebfecb84f64a069897515042a44".to_string(),
         name: "Google Calendar".to_string(),
@@ -34,7 +28,6 @@ fn build_request_plugin_install_elicitation_request_uses_expected_shape() {
         "codex-apps",
         "thread-1".to_string(),
         "turn-1".to_string(),
-        &args,
         "Plan and reference events from your calendar",
         &connector,
     );
@@ -74,12 +67,6 @@ fn build_request_plugin_install_elicitation_request_uses_expected_shape() {
 
 #[test]
 fn build_request_plugin_install_elicitation_request_injects_plugin_metadata() {
-    let args = RequestPluginInstallArgs {
-        tool_type: DiscoverableToolType::Plugin,
-        action_type: DiscoverableToolAction::Install,
-        tool_id: "sample@openai-curated-remote".to_string(),
-        suggest_reason: "Use the sample plugin's skills and MCP server".to_string(),
-    };
     let plugin = DiscoverableTool::Plugin(Box::new(DiscoverablePluginInfo {
         id: "sample@openai-curated-remote".to_string(),
         remote_plugin_id: Some("plugins~Plugin_sample".to_string()),
@@ -94,7 +81,6 @@ fn build_request_plugin_install_elicitation_request_injects_plugin_metadata() {
         "codex-apps",
         "thread-1".to_string(),
         "turn-1".to_string(),
-        &args,
         "Use the sample plugin's skills and MCP server",
         &plugin,
     );
@@ -149,12 +135,8 @@ fn build_request_plugin_install_meta_uses_expected_shape() {
         is_enabled: true,
         plugin_display_names: Vec::new(),
     }));
-    let meta = build_request_plugin_install_meta(
-        DiscoverableToolType::Connector,
-        DiscoverableToolAction::Install,
-        "Find and reference emails from your inbox",
-        &connector,
-    );
+    let meta =
+        build_request_plugin_install_meta("Find and reference emails from your inbox", &connector);
 
     assert_eq!(
         meta,