Load cloud requirements for agent identity (#19708)

## Why

Agent Identity sessions can represent Business and Enterprise ChatGPT
workspaces, but cloud requirements were skipped before fetch. That meant
workspace-managed requirements were not loaded for Agent Identity even
when the JWT carried the same account identity and plan information that
normal ChatGPT token auth exposes.

This PR now sits on top of the Agent Identity stack through
[#19764](https://github.com/openai/codex/pull/19764). Because
[#19763](https://github.com/openai/codex/pull/19763) moved task
registration into Agent Identity auth loading, cloud requirements no
longer needs a separate runtime-initialization step before building the
backend client.

## What changed

- Stop skipping `CodexAuth::AgentIdentity` in the cloud requirements
loader.
- Share the cloud requirements eligibility check between startup load
and background cache refresh.
- Rely on eagerly loaded Agent Identity auth so backend requests can
attach task-scoped `AgentAssertion` headers.
- Decode Agent Identity JWT `plan_type` as the auth-layer plan type,
then convert it through a shared `auth::PlanType` -> `account::PlanType`
mapping.
- Add the missing serde alias for the `education` plan string and add
coverage for raw Agent Identity plan aliases such as `hc` and
`education`.

## Testing

- `cargo test -p codex-agent-identity -p codex-login -p
codex-cloud-requirements -p codex-protocol`

codex-rs/login/src/auth/manager.rs

@@ -39,7 +39,6 @@ use crate::token_data::parse_jwt_expiration;
 use codex_client::CodexHttpClient;
 use codex_config::types::AuthCredentialsStoreMode;
 use codex_protocol::account::PlanType as AccountPlanType;
-use codex_protocol::auth::KnownPlan as InternalKnownPlan;
 use codex_protocol::auth::PlanType as InternalPlanType;
 use codex_protocol::auth::RefreshTokenFailedError;
 use codex_protocol::auth::RefreshTokenFailedReason;
@@ -380,29 +379,10 @@ impl CodexAuth {
             return Some(auth.plan_type());
         }
 
-        let map_known = |kp: &InternalKnownPlan| match kp {
-            InternalKnownPlan::Free => AccountPlanType::Free,
-            InternalKnownPlan::Go => AccountPlanType::Go,
-            InternalKnownPlan::Plus => AccountPlanType::Plus,
-            InternalKnownPlan::Pro => AccountPlanType::Pro,
-            InternalKnownPlan::ProLite => AccountPlanType::ProLite,
-            InternalKnownPlan::Team => AccountPlanType::Team,
-            InternalKnownPlan::SelfServeBusinessUsageBased => {
-                AccountPlanType::SelfServeBusinessUsageBased
-            }
-            InternalKnownPlan::Business => AccountPlanType::Business,
-            InternalKnownPlan::EnterpriseCbpUsageBased => AccountPlanType::EnterpriseCbpUsageBased,
-            InternalKnownPlan::Enterprise => AccountPlanType::Enterprise,
-            InternalKnownPlan::Edu => AccountPlanType::Edu,
-        };
-
         self.get_current_token_data().map(|t| {
             t.id_token
                 .chatgpt_plan_type
-                .map(|pt| match pt {
-                    InternalPlanType::Known(k) => map_known(&k),
-                    InternalPlanType::Unknown(_) => AccountPlanType::Unknown,
-                })
+                .map(AccountPlanType::from)
                 .unwrap_or(AccountPlanType::Unknown)
         })
     }