[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/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -17662,6 +17662,32 @@
             "title": "ImageViewThreadItem",
             "type": "object"
           },
+          {
+            "properties": {
+              "durationMs": {
+                "format": "uint64",
+                "minimum": 0.0,
+                "type": "integer"
+              },
+              "id": {
+                "type": "string"
+              },
+              "type": {
+                "enum": [
+                  "sleep"
+                ],
+                "title": "SleepThreadItemType",
+                "type": "string"
+              }
+            },
+            "required": [
+              "durationMs",
+              "id",
+              "type"
+            ],
+            "title": "SleepThreadItem",
+            "type": "object"
+          },
           {
             "properties": {
               "id": {