[Subtask] Add Semantic Model create and load operations in Gravitino Core

### Describe the subtask

Add managed Core operations for creating and loading Semantic Models. Create performs Java-native definition and source validation, constructs entity and audit metadata, and persists it through the entity store. Load returns the persisted Semantic Model without revalidating external sources.

This subtask includes dispatcher and environment integration with focused unit and JDBC tests. List, alter, drop, REST APIs, and clients are out of scope.

### Parent issue

#12209

Start with parent issue #12209 and the existing Core dispatcher and entity-store integration points. Run the focused unit and JDBC tests while implementing the scoped create and load behavior; done means validated definitions and sources are persisted with entity and audit metadata, and load returns the persisted Semantic Model without external-source revalidation.

Written by the indexing model from the issue text.