[codex] Support multiple forced ChatGPT workspaces (#18161)

## Summary

This change lets `forced_chatgpt_workspace_id` accept multiple workspace
IDs instead of a single value.

It keeps the existing config key name, adds backward-compatible parsing
for a single string in `config.toml`, and normalizes the setting into an
allowed workspace list across login enforcement, app-server config
surfaces, and local ChatGPT auth helpers.

## Why

Workspace-restricted deployments may need to allow more than one ChatGPT
workspace without dropping the guardrail entirely.

## Server-side impact

Codex's local server and app-server protocol needed changes because they
previously assumed a single workspace ID. The local login flow now
matches the auth backend interface by sending the allowed workspace list
as a single comma-separated `allowed_workspace_id` query parameter.

## Validation

This was tested with:

- A single workspace config
- With multi-workspace configs
- With multiple workspaces in the config
- The user only being a part of a subset of them

All were successful.

Automated coverage:

- `cargo test -p codex-login`
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-tui local_chatgpt_auth`
- `cargo test --locked -p codex-app-server
login_account_chatgpt_includes_forced_workspace_allowlist_query_param`

codex-rs/cli/src/login.rs

@@ -115,7 +115,7 @@ fn print_login_server_start(actual_port: u16, auth_url: &str) {
 
 pub async fn login_with_chatgpt(
     codex_home: PathBuf,
-    forced_chatgpt_workspace_id: Option<String>,
+    forced_chatgpt_workspace_id: Option<Vec<String>>,
     cli_auth_credentials_store_mode: AuthCredentialsStoreMode,
 ) -> std::io::Result<()> {
     let opts = ServerOptions::new(