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/mod.rs

@@ -54,8 +54,6 @@ use chrono::Utc;
 use codex_analytics::AnalyticsEventsClient;
 use codex_analytics::SubAgentThreadStartedInput;
 use codex_analytics::TurnCodexErrorFact;
-use codex_app_server_protocol::McpServerElicitationRequest;
-use codex_app_server_protocol::McpServerElicitationRequestParams;
 use codex_config::types::AuthKeyringBackendKind;
 use codex_config::types::OAuthCredentialsStoreMode;
 use codex_exec_server::Environment;
@@ -87,6 +85,7 @@ use codex_otel::current_span_w3c_trace_context;
 use codex_otel::set_parent_from_w3c_trace_context;
 use codex_protocol::SessionId;
 use codex_protocol::ThreadId;
+use codex_protocol::approvals::ElicitationRequest;
 use codex_protocol::approvals::ElicitationRequestEvent;
 use codex_protocol::approvals::ExecPolicyAmendment;
 use codex_protocol::approvals::NetworkPolicyAmendment;