feat(transport): 暴露同格式 provider 的细分失败原因并在时间线展示

- claude_code/kiro/vertex policy 新增 transport_unsupported_reason_with_network 变体，返回具体失败码
- planner candidate_metadata 在 request_pair 写入同格式 provider 的细分 transport 原因
- 前端请求时间线优先展示 transport_diagnostics 的细分原因而非通用 transport_unsupported

crates/aether-provider-transport/src/kiro.rs

@@ -17,6 +17,7 @@ pub use converter::convert_claude_messages_to_conversation_state;
 pub use credentials::{generate_machine_id, normalize_machine_id, KiroAuthConfig};
 pub use headers::{build_generate_assistant_headers, AWS_EVENTSTREAM_CONTENT_TYPE};
 pub use policy::{
+    local_kiro_request_transport_unsupported_reason_with_network,
     supports_local_kiro_request_transport, supports_local_kiro_request_transport_with_network,
 };
 pub use refresh::KiroOAuthRefreshAdapter;