Distinguish policy-restricted capacity failures from server overloads (#57452)

## What changed

- Recognize `policy_capacity_unavailable` in HTTP and streaming API errors and expose it as `policyCapacityUnavailable` through the app-server protocol and generated bindings.
- Display “Policy-restricted capacity is unavailable. Please try again later.” while preserving overload retry behavior based on `Retry-After`.
- Handle the new error in TUI overload rendering, reviewer retries, and error telemetry.

## Testing

Add coverage for HTTP and streaming error classification, retry advice, WebSocket terminal error attribution, protocol serialization, and TUI rendering with and without an upstream message.

GitOrigin-RevId: 1b9536fba1e11cc891dcf689aee35410d82f5412

codex-rs/codex-api/src/error.rs

@@ -57,6 +57,9 @@ pub enum ApiError {
     },
     #[error("Flex capacity unavailable.")]
     FlexUnavailable,
+    /// Server-confirmed capacity failure on a policy-restricted route.
+    #[error("policy-restricted capacity unavailable")]
+    PolicyCapacityUnavailable { retry_after: Option<RetryAfter> },
     #[error("server overloaded")]
     ServerOverloaded { retry_after: Option<RetryAfter> },
 }