Add user_input_requested_during_turn to MCP turn metadata (#22237)

## Why
- Similar change as https://github.com/openai/codex/pull/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`

codex-rs/core/src/session/mcp.rs

@@ -154,6 +154,9 @@ impl Session {
             id,
             request,
         });
+        turn_context
+            .turn_metadata_state
+            .mark_user_input_requested_during_turn();
         self.send_event(turn_context, event).await;
         rx_response.await.ok()
     }

codex-rs/core/src/session/mod.rs

@@ -2289,6 +2289,9 @@ impl Session {
             turn_id: turn_context.sub_id.clone(),
             questions: args.questions,
         });
+        turn_context
+            .turn_metadata_state
+            .mark_user_input_requested_during_turn();
         self.send_event(turn_context, event).await;
         rx_response.await.ok()
     }