feat(app-server): add ThreadItem::DynamicToolCall (#12732)

Previously, clients would call `thread/start` with dynamic_tools set,
and when a model invokes a dynamic tool, it would just make the
server->client `item/tool/call` request and wait for the client's
response to complete the tool call. This works, but it doesn't have an
`item/started` or `item/completed` event.

Now we are doing this:
- [new] emit `item/started` with `DynamicToolCall` populated with the
call arguments
- send an `item/tool/call` server request
- [new] once the client responds, emit `item/completed` with
`DynamicToolCall` populated with the response.

Also, with `persistExtendedHistory: true`, dynamic tool calls are now
reconstructable in `thread/read` and `thread/resume` as
`ThreadItem::DynamicToolCall`.

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

@@ -345,6 +345,58 @@
       ],
       "type": "string"
     },
+    "DynamicToolCallOutputContentItem": {
+      "oneOf": [
+        {
+          "properties": {
+            "text": {
+              "type": "string"
+            },
+            "type": {
+              "enum": [
+                "inputText"
+              ],
+              "title": "InputTextDynamicToolCallOutputContentItemType",
+              "type": "string"
+            }
+          },
+          "required": [
+            "text",
+            "type"
+          ],
+          "title": "InputTextDynamicToolCallOutputContentItem",
+          "type": "object"
+        },
+        {
+          "properties": {
+            "imageUrl": {
+              "type": "string"
+            },
+            "type": {
+              "enum": [
+                "inputImage"
+              ],
+              "title": "InputImageDynamicToolCallOutputContentItemType",
+              "type": "string"
+            }
+          },
+          "required": [
+            "imageUrl",
+            "type"
+          ],
+          "title": "InputImageDynamicToolCallOutputContentItem",
+          "type": "object"
+        }
+      ]
+    },
+    "DynamicToolCallStatus": {
+      "enum": [
+        "inProgress",
+        "completed",
+        "failed"
+      ],
+      "type": "string"
+    },
     "FileUpdateChange": {
       "properties": {
         "diff": {
@@ -1207,6 +1259,59 @@
           "title": "McpToolCallThreadItem",
           "type": "object"
         },
+        {
+          "properties": {
+            "arguments": true,
+            "contentItems": {
+              "items": {
+                "$ref": "#/definitions/DynamicToolCallOutputContentItem"
+              },
+              "type": [
+                "array",
+                "null"
+              ]
+            },
+            "durationMs": {
+              "description": "The duration of the dynamic tool call in milliseconds.",
+              "format": "int64",
+              "type": [
+                "integer",
+                "null"
+              ]
+            },
+            "id": {
+              "type": "string"
+            },
+            "status": {
+              "$ref": "#/definitions/DynamicToolCallStatus"
+            },
+            "success": {
+              "type": [
+                "boolean",
+                "null"
+              ]
+            },
+            "tool": {
+              "type": "string"
+            },
+            "type": {
+              "enum": [
+                "dynamicToolCall"
+              ],
+              "title": "DynamicToolCallThreadItemType",
+              "type": "string"
+            }
+          },
+          "required": [
+            "arguments",
+            "id",
+            "status",
+            "tool",
+            "type"
+          ],
+          "title": "DynamicToolCallThreadItem",
+          "type": "object"
+        },
         {
           "properties": {
             "agentsStates": {