[codex] Add interruptible sleep tool (#28429)

## Why

Models sometimes need to pause briefly while waiting for external work,
but using a shell command for that delay ties the wait to a process and
does not naturally resume when new turn input arrives.

## What changed

- add a built-in `sleep` tool behind the under-development `sleep_tool`
feature
- accept a bounded `duration_ms` argument, matching the millisecond
convention used by unified exec
- end the sleep early when either steered user input or mailbox input
arrives
- include elapsed wall-clock time in completed and interrupted outputs
- emit a dedicated core `SleepItem` through `item/started` and
`item/completed`
- expose the sleep item as app-server v2 `ThreadItem::Sleep` and retain
it in reconstructed thread history
- regenerate the configuration schema for the new feature flag
- regenerate app-server JSON and TypeScript schema fixtures

## Test plan

- `just test -p codex-core sleep_tool_follows_feature_gate`
- `just test -p codex-core any_new_input_interrupts_sleep`
- `just test -p codex-app-server-protocol`
- `just test -p codex-app-server
sleep_emits_started_and_completed_items`

codex-rs/features/src/lib.rs

@@ -201,6 +201,8 @@ pub enum Feature {
     Goals,
     /// Add current context-window metadata to model-visible context.
     TokenBudget,
+    /// Expose an input-interruptible sleep tool.
+    SleepTool,
     /// Route MCP tool approval prompts through the MCP elicitation request path.
     ToolCallMcpElicitation,
     /// Prompt Codex Apps connector auth failures through MCP URL elicitations.
@@ -1157,6 +1159,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::SleepTool,
+        key: "sleep_tool",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::CollaborationModes,
         key: "collaboration_modes",