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/v2/CommandExecTerminateParams.json

@@ -0,0 +1,15 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "description": "Terminate a running `command/exec` session.",
+  "properties": {
+    "processId": {
+      "description": "Client-supplied, connection-scoped `processId` from the original `command/exec` request.",
+      "type": "string"
+    }
+  },
+  "required": [
+    "processId"
+  ],
+  "title": "CommandExecTerminateParams",
+  "type": "object"
+}