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
> }
> }
...