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/CommandExecTerminateResponse.json

@@ -0,0 +1,6 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "description": "Empty success response for `command/exec/terminate`.",
+  "title": "CommandExecTerminateResponse",
+  "type": "object"
+}