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/v2/ThreadReadResponse.json

@@ -444,6 +444,114 @@
       ],
       "type": "object"
     },
+    "FunctionCallOutputBody": {
+      "anyOf": [
+        {
+          "type": "string"
+        },
+        {
+          "items": {
+            "$ref": "#/definitions/FunctionCallOutputContentItem"
+          },
+          "type": "array"
+        }
+      ]
+    },
+    "FunctionCallOutputContentItem": {
+      "description": "Responses API compatible content items that can be returned by a tool call. This is a subset of ContentItem with the types we support as function call outputs.",
+      "oneOf": [
+        {
+          "properties": {
+            "text": {
+              "type": "string"
+            },
+            "type": {
+              "enum": [
+                "input_text"
+              ],
+              "title": "InputTextFunctionCallOutputContentItemType",
+              "type": "string"
+            }
+          },
+          "required": [
+            "text",
+            "type"
+          ],
+          "title": "InputTextFunctionCallOutputContentItem",
+          "type": "object"
+        },
+        {
+          "properties": {
+            "detail": {
+              "anyOf": [
+                {
+                  "$ref": "#/definitions/ImageDetail"
+                },
+                {
+                  "type": "null"
+                }
+              ]
+            },
+            "image_url": {
+              "type": "string"
+            },
+            "type": {
+              "enum": [
+                "input_image"
+              ],
+              "title": "InputImageFunctionCallOutputContentItemType",
+              "type": "string"
+            }
+          },
+          "required": [
+            "image_url",
+            "type"
+          ],
+          "title": "InputImageFunctionCallOutputContentItem",
+          "type": "object"
+        },
+        {
+          "properties": {
+            "audio_url": {
+              "type": "string"
+            },
+            "type": {
+              "enum": [
+                "input_audio"
+              ],
+              "title": "InputAudioFunctionCallOutputContentItemType",
+              "type": "string"
+            }
+          },
+          "required": [
+            "audio_url",
+            "type"
+          ],
+          "title": "InputAudioFunctionCallOutputContentItem",
+          "type": "object"
+        },
+        {
+          "properties": {
+            "encrypted_content": {
+              "type": "string"
+            },
+            "type": {
+              "enum": [
+                "encrypted_content"
+              ],
+              "title": "EncryptedContentFunctionCallOutputContentItemType",
+              "type": "string"
+            }
+          },
+          "required": [
+            "encrypted_content",
+            "type"
+          ],
+          "title": "EncryptedContentFunctionCallOutputContentItem",
+          "type": "object"
+        }
+      ]
+    },
     "GitInfo": {
       "properties": {
         "branch": {
@@ -1262,6 +1370,40 @@
           "title": "AgentMessageThreadItem",
           "type": "object"
         },
+        {
+          "properties": {
+            "id": {
+              "type": "string"
+            },
+            "name": {
+              "type": "string"
+            },
+            "namespace": {
+              "type": [
+                "string",
+                "null"
+              ]
+            },
+            "output": {
+              "$ref": "#/definitions/FunctionCallOutputBody"
+            },
+            "type": {
+              "enum": [
+                "functionCallOutput"
+              ],
+              "title": "FunctionCallOutputThreadItemType",
+              "type": "string"
+            }
+          },
+          "required": [
+            "id",
+            "name",
+            "output",
+            "type"
+          ],
+          "title": "FunctionCallOutputThreadItem",
+          "type": "object"
+        },
         {
           "description": "EXPERIMENTAL - proposed plan item content. The completed plan item is authoritative and may not match the concatenation of `PlanDelta` text.",
           "properties": {