Support standalone tool outputs in turn/start (#41002)

## What changed

- Add an optional `toolOutput` to `turn/start` for starting or steering a turn with a named function-call output instead of user input.
- Emit standalone outputs as `functionCallOutput` thread items, persist them in durable history, and include them in resumed threads.
- Validate that tool outputs have a name, are not combined with nonempty `input`, satisfy the text-size limit, and use supported image URLs.

## Testing

- Cover standalone tool-output notifications, model input, and resumed history in both legacy and paginated history modes.
- Cover the text-size limit for structured tool output.

GitOrigin-RevId: a1eeec7b9b7de67b1a1c5e829a54552b85ec118b

codex-rs/app-server-protocol/schema/json/ClientRequest.json

@@ -5395,6 +5395,16 @@
         "threadId": {
           "type": "string"
         },
+        "toolOutput": {
+          "anyOf": [
+            {
+              "$ref": "#/definitions/TurnToolOutput"
+            },
+            {
+              "type": "null"
+            }
+          ]
+        },
         "turnTrigger": {
           "description": "Optional source classification for the caller that starts this turn. Ignored when this request steers an already-active turn.",
           "type": [
@@ -5438,6 +5448,27 @@
       ],
       "type": "object"
     },
+    "TurnToolOutput": {
+      "properties": {
+        "name": {
+          "type": "string"
+        },
+        "namespace": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "output": {
+          "$ref": "#/definitions/FunctionCallOutputBody"
+        }
+      },
+      "required": [
+        "name",
+        "output"
+      ],
+      "type": "object"
+    },
     "UserInput": {
       "oneOf": [
         {