[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/app-server-protocol/src/protocol/v1.rs

@@ -22,6 +22,7 @@ use serde::Serialize;
 use ts_rs::TS;
 
 use crate::protocol::common::AuthMode;
+use crate::protocol::v2::ForcedChatgptWorkspaceIds;
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Default, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
@@ -201,7 +202,7 @@ pub struct UserSavedConfig {
     pub approval_policy: Option<AskForApproval>,
     pub sandbox_mode: Option<SandboxMode>,
     pub sandbox_settings: Option<SandboxSettings>,
-    pub forced_chatgpt_workspace_id: Option<String>,
+    pub forced_chatgpt_workspace_id: Option<ForcedChatgptWorkspaceIds>,
     pub forced_login_method: Option<ForcedLoginMethod>,
     pub model: Option<String>,
     pub model_reasoning_effort: Option<ReasoningEffort>,