Add user_input_requested_during_turn to MCP turn metadata

[mchen-oai]

Why

• Similar change as Add model and reasoning effort to MCP turn metadata #21219
• Without change: MCP tool calls receive _meta["x-codex-turn-metadata"] with various key values.
• Issue: MCP servers currently do not know if user input was requested during the turn (Ex: Model decides to prompt the user for approval mid-turn before making a possibly risky tool call). MCP servers may want to know this when tracking latency metrics because these instances are inflated.

What Changed

• With change: MCP turn metadata now includes user_input_requested_during_turn when a model-visible request_user_input call happened earlier in the turn, propagated in _meta["x-codex-turn-metadata"].
• mark_turn_user_input_requested() is called when user input is requested through either MCP elicitation (mcp.rs) or the request_user_input tool (mod.rs).
• MCP tool call _meta is now built immediately before execution (mcp_tool_call.rs) so user input requested earlier in the same turn, including within the same tool call via elicitation, is reflected in the metadata.
• Normal /responses turn metadata headers are unchanged.

Verification

• codex-rs/core/src/session/mcp_tests.rs
• codex-rs/core/src/tools/handlers/request_user_input_tests.rs
• codex-rs/core/src/turn_metadata_tests.rs
• codex-rs/core/tests/suite/search_tool.rs