feat(gateway): harden failover and payload handling

Retry pre-response transport failures across candidates with an explicit stop policy, and propagate end-to-end timing into usage records and UI diagnostics.

Remove legacy body, import, cookie, PII, and tunnel replay caps while preserving optional operator-configured gateway limits.

apps/aether-gateway/src/execution_runtime/server.rs

@@ -370,6 +370,9 @@ impl IntoResponse for ExecutionRuntimeAppError {
                 | ExecutionRuntimeTransportError::UnsupportedTransportProfile(_)
                 | ExecutionRuntimeTransportError::BodyEncode(_),
             ) => StatusCode::BAD_REQUEST,
+            ExecutionRuntimeServerError::Transport(
+                ExecutionRuntimeTransportError::UpstreamHttpStatus { status_code, .. },
+            ) => StatusCode::from_u16(status_code).unwrap_or(StatusCode::BAD_GATEWAY),
             ExecutionRuntimeServerError::Transport(
                 ExecutionRuntimeTransportError::ClientBuild(_)
                 | ExecutionRuntimeTransportError::BrowserClientBuild(_)