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/CommandExecResponse.json

@@ -1,14 +1,18 @@
 {
   "$schema": "http://json-schema.org/draft-07/schema#",
+  "description": "Final buffered result for `command/exec`.",
   "properties": {
     "exitCode": {
+      "description": "Process exit code.",
       "format": "int32",
       "type": "integer"
     },
     "stderr": {
+      "description": "Buffered stderr capture.\n\nEmpty when stderr was streamed via `command/exec/outputDelta`.",
       "type": "string"
     },
     "stdout": {
+      "description": "Buffered stdout capture.\n\nEmpty when stdout was streamed via `command/exec/outputDelta`.",
       "type": "string"
     }
   },