add turn items view to app-server turns (#21063)

## Why

`Turn.items` currently overloads an empty array to mean either that no
items exist or that the server intentionally did not load them for this
response. That ambiguity blocks future lazy-loading work where clients
need to distinguish unloaded, summary, and fully hydrated turn payloads.

## What changed

- add a new `TurnItemsView` enum with `notLoaded`, `summary`, and `full`
variants
- add required `itemsView` metadata to app-server `Turn` payloads
- mark reconstructed persisted history as `full` and live shell-style
turn payloads as `notLoaded`
- keep current `thread/turns/list` behavior unchanged and document that
it still returns `full` turns today
- regenerate the JSON and TypeScript protocol fixtures

## Verification

- `just write-app-server-schema`
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-app-server thread_read_can_include_turns`
- `cargo test -p codex-app-server
thread_turns_list_can_page_backward_and_forward`
- `cargo test -p codex-app-server
thread_resume_rejects_history_when_thread_is_running`
- `just fix -p codex-app-server-protocol`
- `just fix -p codex-app-server`
- `just fmt`

codex-rs/app-server-protocol/schema/json/v2/ThreadMetadataUpdateResponse.json

@@ -1675,12 +1675,21 @@
           "type": "string"
         },
         "items": {
-          "description": "Only populated on a `thread/resume` or `thread/fork` response. For all other responses and notifications returning a Turn, the items field will be an empty list.",
+          "description": "Thread items currently included in this turn payload.",
           "items": {
             "$ref": "#/definitions/ThreadItem"
           },
           "type": "array"
         },
+        "itemsView": {
+          "allOf": [
+            {
+              "$ref": "#/definitions/TurnItemsView"
+            }
+          ],
+          "default": "full",
+          "description": "Describes how much of `items` has been loaded for this turn."
+        },
         "startedAt": {
           "description": "Unix timestamp (in seconds) when the turn started.",
           "format": "int64",
@@ -1728,6 +1737,31 @@
       ],
       "type": "object"
     },
+    "TurnItemsView": {
+      "oneOf": [
+        {
+          "description": "`items` was not loaded for this turn. The field is intentionally empty.",
+          "enum": [
+            "notLoaded"
+          ],
+          "type": "string"
+        },
+        {
+          "description": "`items` contains only a display summary for this turn.",
+          "enum": [
+            "summary"
+          ],
+          "type": "string"
+        },
+        {
+          "description": "`items` contains every ThreadItem available from persisted app-server history for this turn.",
+          "enum": [
+            "full"
+          ],
+          "type": "string"
+        }
+      ]
+    },
     "TurnStatus": {
       "enum": [
         "completed",