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/Cargo.lock

@@ -4020,7 +4020,6 @@ dependencies = [
 name = "codex-tools"
 version = "0.0.0"
 dependencies = [
- "codex-app-server-protocol",
  "codex-code-mode",
  "codex-connectors",
  "codex-features",