Test: release Go-proxy RESTful contract tests verified passing in Go mode (#17468)

### Summary

Aligns Go and Python error codes/messages so both backends honor the
same RESTful API contract, removing implementation-specific error leaks
(MySQL errors, `ValueError`, `AttributeError`, Gin validator format) in
favor of clean business error codes.

**Chat list** — invalid `orderby` now returns code 101 (was: raw Python
`AttributeError` code 100); invalid `page`/`page_size` values fall back
to defaults (was: raw `ValueError`/`ProgrammingError` code 100).

**Dataset create/update/delete** — adds UUID validation (101),
extra-field rejection (101), duplicate-id detection (101), content-type
/ JSON-syntax / object-shape checks (101), and "lacks permission" for
nonexistent datasets (IDOR). Create auto-deduplicates dataset names.
Pagerank updates tolerate a missing ES index. List response includes
`parser_config` and `pagerank`.

**Session list/update** — adds filtering, sorting, and pagination
support. Empty payloads are valid no-ops. Authorization errors map to
code 109.

**Chunk list** — doc object uses Python key names (`chunk_count`,
`dataset_id`, `chunk_method`, run text status). Add validates list
element types.

**Document update** — adds `chunk_method` alias, pydantic-style Field
error messages, metadata index auto-create with refresh, and "These
documents do not belong to dataset" messages. List validates
`metadata_condition` and reports ownership errors for unmatched name/id
filters.

**Search completion** — `kb_ids` ownership failure returns code 102
instead of 109.

Released 31 contract tests from `GO_ONLY_SKIPS` (all verified passing on
both Go and Python backends with real LLM keys).

internal/common/error_code.go

@@ -89,6 +89,20 @@ func (e ErrorCode) Message() string {
 	return "Unknown error"
 }
 
+// CodedError pairs a business ErrorCode with a message so handlers can return
+// the established {code, message} contract (HTTP 200) instead of a generic
+// HTTP 500 for expected domain failures.
+type CodedError struct {
+	Code    ErrorCode
+	Message string
+}
+
+func (e *CodedError) Error() string { return e.Message }
+
+func NewCodedError(code ErrorCode, message string) *CodedError {
+	return &CodedError{Code: code, Message: message}
+}
+
 var (
 	ErrInvalidToken = errors.New("invalid token")
 	ErrNotAdmin     = errors.New("user is not admin")