chore: import upstream snapshot with attribution

docs/developer_guides/appendix_d_error_code.md

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+## Appendix D. Error Code
+
+> **⚠️ Deprecated.** This appendix documents the legacy `commonpb.ErrorCode`
+> enum (Milvus 2.0-era), which is no longer the source of truth for error codes.
+> Current error codes are the `merr` sentinel codes (e.g. `ParameterInvalid`
+> = 1100, `ServiceInternal` = 5, `FunctionFailed` = 2400) carried in
+> `commonpb.Status.Code`. The canonical list is the sentinel definitions in
+> [`pkg/util/merr/errors.go`](../../pkg/util/merr/errors.go); see also
+> [error_handling_guide.md](../dev/error_handling_guide.md) and
+> [error_sentinel_convention.md](../dev/error_sentinel_convention.md).
+
+**ErrorCode**
+
+```protobuf
+enum ErrorCode {
+    Success = 0;
+    UnexpectedError = 1;
+    ConnectFailed = 2;
+    PermissionDenied = 3;
+    CollectionNotExists = 4;
+    IllegalArgument = 5;
+    IllegalDimension = 7;
+    IllegalIndexType = 8;
+    IllegalCollectionName = 9;
+    IllegalTOPK = 10;
+    IllegalRowRecord = 11;
+    IllegalVectorID = 12;
+    IllegalSearchResult = 13;
+    FileNotFound = 14;
+    MetaFailed = 15;
+    CacheFailed = 16;
+    CannotCreateFolder = 17;
+    CannotCreateFile = 18;
+    CannotDeleteFolder = 19;
+    CannotDeleteFile = 20;
+    BuildIndexError = 21;
+    IllegalNLIST = 22;
+    IllegalMetricType = 23;
+    OutOfMemory = 24;
+    IndexNotExist = 25;
+    EmptyCollection = 26;
+
+    // internal error code.
+    DDRequestRace = 1000;
+}
+```