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/engine.go

@@ -32,6 +32,8 @@ type EngineType string
 const (
 	EngineElasticsearch EngineType = "elasticsearch"
 	EngineInfinity      EngineType = "infinity"
+	EngineOceanBase     EngineType = "oceanbase"
+	EngineSeekDB        EngineType = "seekdb"
 	EngineSereneDB      EngineType = "serenedb"
 )
 
@@ -94,10 +96,16 @@ type DocEngine interface {
 // Type returns the engine type (helper method for runtime type checking)
 // This is a workaround since we can't import elasticsearch or infinity packages directly
 func Type(docEngine DocEngine) EngineType {
-	// Type checking through interface methods is not straightforward
-	// This is a placeholder that should be implemented differently
-	// or rely on configuration to know the type
-	return EngineType("unknown")
+	if docEngine == nil {
+		return EngineType("unknown")
+	}
+	return EngineType(docEngine.GetType())
+}
+
+// IsOceanBaseFamily reports whether a configured engine uses the shared
+// OceanBase/SeekDB SQL implementation.
+func IsOceanBaseFamily(engineName string) bool {
+	return engineName == string(EngineOceanBase) || engineName == string(EngineSeekDB)
 }
 
 type MessageQueue interface {