[codex] [2/4] Generalize plugin suggestion presentation (#28400)

Summary
- Add list-backed and developer-context presentations for plugin
suggestion candidates.
- Let tool planning, install validation, and request-tool copy follow
the selected presentation.
- Keep every production caller on the existing list-backed presentation,
preserving the current list tool, request schema, connector behavior,
and model-visible copy.
- Leave developer-context presentation latent until the final PR in the
stack.

Stack
- 2/3, based on #28399.
- Follow-up: #27704 activates endpoint recommendations.

Validation
- `just test -p codex-core request_plugin_install`
- `just test -p codex-core spec_plan`
- `just fix -p codex-core`
- `just fmt`
- `git diff --check`

codex-rs/core/src/session/tests.rs

@@ -571,9 +571,9 @@ fn test_tool_runtime(session: Arc<Session>, turn_context: Arc<TurnContext>) -> T
     let router = Arc::new(ToolRouter::from_turn_context(
         &turn_context,
         crate::tools::router::ToolRouterParams {
+            tool_suggest_candidates: None,
             mcp_tools: None,
             deferred_mcp_tools: None,
-            discoverable_tools: None,
             extension_tool_executors: Vec::new(),
             dynamic_tools: turn_context.dynamic_tools.as_slice(),
         },
@@ -9600,9 +9600,9 @@ async fn fatal_tool_error_stops_turn_and_reports_error() {
     let router = ToolRouter::from_turn_context(
         &turn_context,
         crate::tools::router::ToolRouterParams {
+            tool_suggest_candidates: None,
             deferred_mcp_tools,
             mcp_tools: Some(tools),
-            discoverable_tools: None,
             extension_tool_executors: Vec::new(),
             dynamic_tools: turn_context.dynamic_tools.as_slice(),
         },