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/tools/src/request_plugin_install.rs

@@ -1,10 +1,5 @@
-use std::collections::BTreeMap;
-
-use codex_app_server_protocol::McpElicitationObjectType;
-use codex_app_server_protocol::McpElicitationSchema;
-use codex_app_server_protocol::McpServerElicitationRequest;
-use codex_app_server_protocol::McpServerElicitationRequestParams;
 use codex_connectors::AppInfo;
+use codex_protocol::approvals::ElicitationRequest;
 use serde::Deserialize;
 use serde::Serialize;
 use serde_json::json;
@@ -54,31 +49,21 @@ pub struct RequestPluginInstallMeta<'a> {
 }
 
 pub fn build_request_plugin_install_elicitation_request(
-    server_name: &str,
-    thread_id: String,
-    turn_id: String,
     suggest_reason: &str,
     tool: &DiscoverableTool,
-) -> McpServerElicitationRequestParams {
+) -> ElicitationRequest {
     let message = suggest_reason.to_string();
 
-    McpServerElicitationRequestParams {
-        thread_id,
-        turn_id: Some(turn_id),
-        server_name: server_name.to_string(),
-        request: McpServerElicitationRequest::Form {
-            meta: Some(json!(build_request_plugin_install_meta(
-                suggest_reason,
-                tool,
-            ))),
-            message,
-            requested_schema: McpElicitationSchema {
-                schema_uri: None,
-                type_: McpElicitationObjectType::Object,
-                properties: BTreeMap::new(),
-                required: None,
-            },
-        },
+    ElicitationRequest::Form {
+        meta: Some(json!(build_request_plugin_install_meta(
+            suggest_reason,
+            tool,
+        ))),
+        message,
+        requested_schema: json!({
+            "type": "object",
+            "properties": {},
+        }),
     }
 }