[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/core/tests/suite/request_plugin_install.rs

@@ -230,9 +230,7 @@ async fn endpoint_mode_injects_candidates_hides_list_and_rejects_invented_ids()
                     call_id,
                     REQUEST_PLUGIN_INSTALL_TOOL_NAME,
                     &serde_json::to_string(&json!({
-                        "tool_type": "plugin",
-                        "action_type": "install",
-                        "tool_id": "invented@openai-curated-remote",
+                        "plugin_id": "invented@openai-curated-remote",
                         "suggest_reason": "Try this"
                     }))?,
                 ),
@@ -312,9 +310,7 @@ async fn endpoint_recommendation_adds_install_identity_only_to_elicitation_metad
                     call_id,
                     REQUEST_PLUGIN_INSTALL_TOOL_NAME,
                     &serde_json::to_string(&json!({
-                        "tool_type": "plugin",
-                        "action_type": "install",
-                        "tool_id": "github@openai-curated-remote",
+                        "plugin_id": "github@openai-curated-remote",
                         "suggest_reason": "Use GitHub for this request"
                     }))?,
                 ),