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/mod.rs

@@ -20,6 +20,7 @@ mod stream_pump;
 pub(crate) mod submission;
 pub(crate) mod sync;
 pub(crate) mod transport;
+mod transport_failure;
 mod windsurf;
 
 pub(crate) use self::chatgpt_web_image::maybe_execute_chatgpt_web_image_sync;
@@ -133,6 +134,10 @@ pub(crate) use transport::{
     execute_sync_plan as execute_execution_runtime_sync_plan, DirectSyncExecutionRuntime,
     DirectUpstreamStreamExecution, ExecutionRuntimeTransportError,
 };
+pub(crate) use transport_failure::{
+    build_transport_error_stop_response, mark_stream_candidate_watchdog_terminal_started,
+    StreamCandidateWatchdogProgress,
+};
 
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub(crate) struct ClientIntent {