Add OceanBase and SeekDB Go document engine (#17780)

## What changed

- add an OceanBase/SeekDB Go document engine using `database/sql` and
the existing MySQL driver
- preserve the Python connector's configuration, physical table names,
schema, index names, and ARRAY/JSON/VECTOR encodings
- implement chunk, memory, document metadata, skill, SQL, full-text,
vector, and fusion search paths
- support `DBMS_HYBRID_SEARCH.SEARCH` behind the existing feature flag,
with SQL fallback only when the package is unavailable
- wire the engine into retrieval, memory, metadata, vector hydration,
and SQL chat flows
- add Python/Go compatibility contracts, SQL mock tests, and an
integration-tagged round-trip test

---------

Co-authored-by: Jin Hai <haijin.chn@gmail.com>

internal/engine/oceanbase/sql.go

@@ -0,0 +1,51 @@
+//
+//  Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+//
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+//  Unless required by applicable law or agreed to in writing, software
+//  distributed under the License is distributed on an "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//  See the License for the specific language governing permissions and
+//  limitations under the License.
+//
+
+package oceanbase
+
+import (
+	"context"
+	"fmt"
+	"regexp"
+	"strings"
+)
+
+var (
+	jsonExtractStringPattern = regexp.MustCompile(`(?i)json_extract_string\s*\(\s*([^,]+?)\s*,\s*([^)]+?)\s*\)`)
+	jsonExtractNullPattern   = regexp.MustCompile(`(?i)json_extract_isnull\s*\(\s*([^,]+?)\s*,\s*([^)]+?)\s*\)`)
+	limitPattern             = regexp.MustCompile(`(?i)\blimit\b`)
+)
+
+// RunSQL executes the read-only SQL produced by the chat SQL-retrieval flow.
+func (e *Engine) RunSQL(ctx context.Context, tableName, sqlText string, kbIDs []string, format string) ([]map[string]interface{}, error) {
+	if tableName != "" {
+		if err := validateIdentifier(tableName); err != nil {
+			return nil, err
+		}
+	}
+	normalized := strings.TrimSpace(strings.TrimSuffix(strings.TrimSpace(sqlText), ";"))
+	normalized = strings.ReplaceAll(normalized, "`", "")
+	normalized = jsonExtractStringPattern.ReplaceAllString(normalized, "JSON_UNQUOTE(JSON_EXTRACT($1, $2))")
+	normalized = jsonExtractNullPattern.ReplaceAllString(normalized, "(JSON_EXTRACT($1, $2) IS NULL)")
+	lower := strings.ToLower(strings.TrimSpace(normalized))
+	if !strings.HasPrefix(lower, "select ") && !strings.HasPrefix(lower, "with ") {
+		return nil, fmt.Errorf("only SELECT and WITH statements are allowed")
+	}
+	if !limitPattern.MatchString(normalized) {
+		normalized += " LIMIT 1024"
+	}
+	return e.queryRows(ctx, normalized)
+}