mcp: keep elicitation requests below app wire types (#29724)

## Why

Core and tools need to request MCP elicitation without constructing
app-server wire payloads. The request should remain a neutral protocol
concept until app-server serializes it for a client.

## What changed

- Switched core and tools to
`codex_protocol::approvals::ElicitationRequest`.
- Derived turn and server context inside core instead of carrying
app-server request types through lower layers.
- Kept the app-server payload unchanged through an explicit boundary
conversion.
- Removed the remaining production app-server-protocol dependency from
tools.

## Stack

This is PR 5 of 6, stacked on [PR
#29723](https://github.com/openai/codex/pull/29723). Review only the
delta from `codex/split-connector-metadata-types`. Next: [PR
#29725](https://github.com/openai/codex/pull/29725).

## Validation

- `codex-core` MCP coverage passed: 87 tests.
- Tools elicitation and app-server round-trip coverage passed.

codex-rs/core/src/tools/handlers/request_plugin_install.rs

@@ -173,15 +173,14 @@ impl RequestPluginInstallHandler {
         let tool_type = tool.tool_type();
 
         let request_id = RequestId::String(format!("request_plugin_install_{call_id}").into());
-        let params = build_request_plugin_install_elicitation_request(
-            CODEX_APPS_MCP_SERVER_NAME,
-            session.thread_id.to_string(),
-            turn.sub_id.clone(),
-            suggest_reason,
-            &tool,
-        );
+        let request = build_request_plugin_install_elicitation_request(suggest_reason, &tool);
         let elicitation = session
-            .request_mcp_server_elicitation(turn.as_ref(), request_id, params)
+            .request_mcp_server_elicitation(
+                turn.as_ref(),
+                CODEX_APPS_MCP_SERVER_NAME.to_string(),
+                request_id,
+                request,
+            )
             .await;
         let response = elicitation.response;
         if let Some(response) = response.as_ref() {