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/session/tests.rs

@@ -84,7 +84,6 @@ use crate::tools::handlers::ShellCommandHandler;
 use crate::tools::registry::ToolExecutor;
 use crate::tools::router::ToolCallSource;
 use crate::turn_diff_tracker::TurnDiffTracker;
-use codex_app_server_protocol::McpElicitationSchema;
 use codex_config::config_toml::ConfigToml;
 use codex_config::config_toml::ProjectConfig;
 use codex_config::permissions_toml::FilesystemPermissionToml;
@@ -377,24 +376,18 @@ async fn request_mcp_server_elicitation_auto_accepts_when_auto_deny_is_enabled()
         .load_full()
         .set_elicitations_auto_deny(/*auto_deny*/ true);
 
-    let requested_schema: McpElicitationSchema = serde_json::from_value(json!({
-        "type": "object",
-        "properties": {},
-    }))
-    .expect("schema should deserialize");
     let response = session
         .request_mcp_server_elicitation(
             turn_context.as_ref(),
+            "codex_apps".to_string(),
             RequestId::String("request-1".into()),
-            McpServerElicitationRequestParams {
-                thread_id: session.thread_id.to_string(),
-                turn_id: Some(turn_context.sub_id.clone()),
-                server_name: "codex_apps".to_string(),
-                request: McpServerElicitationRequest::Form {
-                    meta: None,
-                    message: "Allow this request?".to_string(),
-                    requested_schema,
-                },
+            ElicitationRequest::Form {
+                meta: None,
+                message: "Allow this request?".to_string(),
+                requested_schema: json!({
+                    "type": "object",
+                    "properties": {},
+                }),
             },
         )
         .await;