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/transport.rs

@@ -575,6 +575,8 @@ pub(crate) enum ExecutionRuntimeTransportError {
     BrowserClientBuild(wreq::Error),
     #[error("browser impersonation response body failed: {0}")]
     BrowserBody(String),
+    #[error("{message}")]
+    UpstreamHttpStatus { status_code: u16, message: String },
     #[error("failed to execute upstream request: {0}")]
     UpstreamRequest(String),
     #[error("upstream response {phase} body exceeds {limit_bytes} bytes")]