refactor: route Codex auth through AuthProvider (#18811)

## Summary

This PR moves Codex backend request authentication from direct
bearer-token handling to `AuthProvider`.

The new `codex-auth-provider` crate defines the shared request-auth
trait. `CodexAuth::provider()` returns a provider that can apply all
headers needed for the selected auth mode.

This lets ChatGPT token auth and AgentIdentity auth share the same
callsite path:
- ChatGPT token auth applies bearer auth plus account/FedRAMP headers
where needed.
- AgentIdentity auth applies AgentAssertion plus account/FedRAMP headers
where needed.

Reference old stack: https://github.com/openai/codex/pull/17387/changes

## Callsite Migration

| Area | Change |
| --- | --- |
| backend-client | accepts an `AuthProvider` instead of a raw
token/header |
| chatgpt client/connectors | applies auth through
`CodexAuth::provider()` |
| cloud tasks | keeps Codex-backend gating, applies auth through
provider |
| cloud requirements | uses Codex-backend auth checks and provider
headers |
| app-server remote control | applies provider headers for backend calls
|
| MCP Apps/connectors | gates on `uses_codex_backend()` and keys caches
from generic account getters |
| model refresh | treats AgentIdentity as Codex-backend auth |
| OpenAI file upload path | rejects non-Codex-backend auth before
applying headers |
| core client setup | keeps model-provider auth flow and allows
AgentIdentity through provider-backed OpenAI auth |

## Stack

1. https://github.com/openai/codex/pull/18757: full revert
2. https://github.com/openai/codex/pull/18871: isolated Agent Identity
crate
3. https://github.com/openai/codex/pull/18785: explicit AgentIdentity
auth mode and startup task allocation
4. This PR: migrate Codex backend auth callsites through AuthProvider
5. https://github.com/openai/codex/pull/18904: accept AgentIdentity JWTs
and load `CODEX_AGENT_IDENTITY`

## Testing

Tests: targeted Rust checks, cargo-shear, Bazel lock check, and CI.

codex-rs/protocol/src/account.rs

@@ -35,6 +35,18 @@ impl PlanType {
     pub fn is_business_like(self) -> bool {
         matches!(self, Self::Business | Self::EnterpriseCbpUsageBased)
     }
+
+    pub fn is_workspace_account(self) -> bool {
+        matches!(
+            self,
+            Self::Team
+                | Self::SelfServeBusinessUsageBased
+                | Self::Business
+                | Self::EnterpriseCbpUsageBased
+                | Self::Enterprise
+                | Self::Edu
+        )
+    }
 }
 
 #[cfg(test)]
@@ -84,4 +96,21 @@ mod tests {
         assert_eq!(PlanType::EnterpriseCbpUsageBased.is_business_like(), true);
         assert_eq!(PlanType::Team.is_business_like(), false);
     }
+
+    #[test]
+    fn workspace_account_helper_includes_usage_based_workspace_plans() {
+        assert_eq!(PlanType::Team.is_workspace_account(), true);
+        assert_eq!(
+            PlanType::SelfServeBusinessUsageBased.is_workspace_account(),
+            true
+        );
+        assert_eq!(PlanType::Business.is_workspace_account(), true);
+        assert_eq!(
+            PlanType::EnterpriseCbpUsageBased.is_workspace_account(),
+            true
+        );
+        assert_eq!(PlanType::Enterprise.is_workspace_account(), true);
+        assert_eq!(PlanType::Edu.is_workspace_account(), true);
+        assert_eq!(PlanType::Pro.is_workspace_account(), false);
+    }
 }