Add turn start timestamp to turn metadata

[mchen-oai]

Why

• Without change: MCP tool calls receive _meta["x-codex-turn-metadata"] with session_id and turn_id.
• Issue: MCP servers may want the turn start timestamp to measure internal latency relative to turn start.

What Changed

• With change: turn metadata now includes turn_started_at_unix_ms, which is propagated to MCP tool calls in _meta["x-codex-turn-metadata"].

Verification

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

[github-actions]

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[mchen-oai]

I have read the CLA Document and I hereby sign the CLA