fix(oracle): 简化编辑器选择器并支持 schema 切换 (closes #205)

Oracle/Dameng 连接隐藏多余的 Database 选择器，只显示 Schema 选择器。
查询执行时通过 ALTER SESSION SET CURRENT_SCHEMA 切换 schema 上下文。

src/lib/databaseCapabilities.ts

@@ -77,6 +77,8 @@ export const FIELD_LINEAGE_SUPPORTED_TYPES = new Set<DatabaseType>([
   "gaussdb",
 ]);
 
+export const SINGLE_DATABASE_TYPES = new Set<DatabaseType>(["oracle", "dameng"]);
+
 export const FETCH_FIRST_TYPES = new Set<DatabaseType>(["oracle", "dameng"]);
 
 export const TREE_SCHEMA_TYPES = new Set<DatabaseType>(["postgres", "redshift", "sqlserver", "gaussdb", "jdbc"]);
@@ -110,6 +112,10 @@ export function isSchemaAware(dbType?: DatabaseType): boolean {
   return !!dbType && SCHEMA_AWARE_TYPES.has(dbType);
 }
 
+export function isSingleDatabase(dbType?: DatabaseType): boolean {
+  return !!dbType && SINGLE_DATABASE_TYPES.has(dbType);
+}
+
 export function usesFetchFirst(dbType?: DatabaseType): boolean {
   return !!dbType && FETCH_FIRST_TYPES.has(dbType);
 }