refactor: add namespace-aware SchemaView

[shirly121]

Summary

• add a read-only SchemaView that filters vertex and edge schemas by namespace
• persist namespace metadata through schema DDL parameters, WAL records, and schema serialization
• make GOptPlanner::compilePlan use the default namespace view
• return result<> with ERR_NOT_FOUND when schema labels are unavailable in the selected namespace
• add AP and TP storage tests covering namespace isolation before and after transaction commit

Testing

• cmake --build build --target schema_view_test -j$(sysctl -n hw.ncpu)
• ctest --test-dir build -R "^schema_view_test$" --output-on-failure

Fixes #716