chore: add codex debug app-server tooling

[celia-oai]

codex debug app-server forwards the message through codex-app-server-test-client’s send_message_v2 library entry point, using std::env::current_exe() to resolve the codex binary.

for how it looks like, see:

celia@com-92114 codex-rs % cargo build -p codex-cli && target/debug/codex debug app-server --help                       
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.34s
Tooling: helps debug the app server

Usage: codex debug app-server [OPTIONS] <COMMAND>

Commands:
  send-message-v2  
  help             Print this message or the help of the given subcommand(s)

and

celia@com-92114 codex-rs % cargo build -p codex-cli && target/debug/codex debug app-server send-message-v2 "hello world"
   Compiling codex-cli v0.0.0 (/Users/celia/code/codex/codex-rs/cli)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.38s
> {
>   "method": "initialize",
>   "id": "f8ba9f60-3a49-4ea9-81d6-4ab6853e3954",
>   "params": {
>     "clientInfo": {
>       "name": "codex-toy-app-server",
>       "title": "Codex Toy App Server",
>       "version": "0.0.0"
>     },
>     "capabilities": {
>       "experimentalApi": true
>     }
>   }
> }
< {
<   "id": "f8ba9f60-3a49-4ea9-81d6-4ab6853e3954",
<   "result": {
<     "userAgent": "codex-toy-app-server/0.0.0 (Mac OS 26.2.0; arm64) vscode/2.4.27 (codex-toy-app-server; 0.0.0)"
<   }
< }
< initialize response: InitializeResponse { user_agent: "codex-toy-app-server/0.0.0 (Mac OS 26.2.0; arm64) vscode/2.4.27 (codex-toy-app-server; 0.0.0)" }
> {
>   "method": "thread/start",
>   "id": "203f1630-beee-4e60-b17b-9eff16b1638b",
>   "params": {
>     "model": null,
>     "modelProvider": null,
>     "cwd": null,
>     "approvalPolicy": null,
>     "sandbox": null,
>     "config": null,
>     "baseInstructions": null,
>     "developerInstructions": null,
>     "personality": null,
>     "ephemeral": null,
>     "dynamicTools": null,
>     "mockExperimentalField": null,
>     "experimentalRawEvents": false
>   }
> }
...