app-server: Add streaming and tty/pty capabilities to command/exec (#13640)

* Add an ability to stream stdin, stdout, and stderr
* Streaming of stdout and stderr has a configurable cap for total amount
of transmitted bytes (with an ability to disable it)
* Add support for overriding environment variables
* Add an ability to terminate running applications (using
`command/exec/terminate`)
* Add TTY/PTY support, with an ability to resize the terminal (using
`command/exec/resize`)

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

@@ -670,6 +670,57 @@
         }
       ]
     },
+    "CommandExecOutputDeltaNotification": {
+      "description": "Base64-encoded output chunk emitted for a streaming `command/exec` request.\n\nThese notifications are connection-scoped. If the originating connection closes, the server terminates the process.",
+      "properties": {
+        "capReached": {
+          "description": "`true` on the final streamed chunk for a stream when `outputBytesCap` truncated later output on that stream.",
+          "type": "boolean"
+        },
+        "deltaBase64": {
+          "description": "Base64-encoded output bytes.",
+          "type": "string"
+        },
+        "processId": {
+          "description": "Client-supplied, connection-scoped `processId` from the original `command/exec` request.",
+          "type": "string"
+        },
+        "stream": {
+          "allOf": [
+            {
+              "$ref": "#/definitions/CommandExecOutputStream"
+            }
+          ],
+          "description": "Output stream for this chunk."
+        }
+      },
+      "required": [
+        "capReached",
+        "deltaBase64",
+        "processId",
+        "stream"
+      ],
+      "type": "object"
+    },
+    "CommandExecOutputStream": {
+      "description": "Stream label for `command/exec/outputDelta` notifications.",
+      "oneOf": [
+        {
+          "description": "stdout stream. PTY mode multiplexes terminal output here.",
+          "enum": [
+            "stdout"
+          ],
+          "type": "string"
+        },
+        {
+          "description": "stderr stream.",
+          "enum": [
+            "stderr"
+          ],
+          "type": "string"
+        }
+      ]
+    },
     "CommandExecutionOutputDeltaNotification": {
       "properties": {
         "delta": {
@@ -3468,6 +3519,27 @@
       "title": "Item/plan/deltaNotification",
       "type": "object"
     },
+    {
+      "description": "Stream base64-encoded stdout/stderr chunks for a running `command/exec` session.",
+      "properties": {
+        "method": {
+          "enum": [
+            "command/exec/outputDelta"
+          ],
+          "title": "Command/exec/outputDeltaNotificationMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/CommandExecOutputDeltaNotification"
+        }
+      },
+      "required": [
+        "method",
+        "params"
+      ],
+      "title": "Command/exec/outputDeltaNotification",
+      "type": "object"
+    },
     {
       "properties": {
         "method": {