Add model and reasoning effort to MCP turn metadata

[mchen-oai]

Why

• Similar change as Add turn start timestamp to turn metadata #19473.
• Without change: MCP tool calls receive _meta["x-codex-turn-metadata"] with session_id, turn_id, and turn_started_at_unix_ms.
• Issue: MCP servers may want the model and reasoning effort to better understand tool-call behavior and latency relative to turn start.

What Changed

• With change: MCP turn metadata now includes model and reasoning_effort, propagated in _meta["x-codex-turn-metadata"].
• Normal /responses turn metadata headers are unchanged.

Verification

• codex-rs/core/src/mcp_tool_call_tests.rs
• codex-rs/core/src/turn_metadata_tests.rs
• codex-rs/core/tests/suite/search_tool.rs

[mchen-oai]

@owenlin0 @mzeng-openai - Added you as reviewers since this is similar to previous PR you reviewed. Thank you!