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/composables/useSchemaOptions.ts

@@ -1,6 +1,6 @@
 import { ref } from "vue";
 import { useConnectionStore } from "@/stores/connectionStore";
-import { isSchemaAware as isSchemaAwareType } from "@/lib/databaseCapabilities";
+import { isSchemaAware as isSchemaAwareType, isSingleDatabase } from "@/lib/databaseCapabilities";
 import * as api from "@/lib/api";
 
 export function useSchemaOptions() {
@@ -18,7 +18,9 @@ export function useSchemaOptions() {
   }
 
   async function loadSchemaOptions(connectionId: string, database: string) {
-    if (!isSchemaAware(connectionId) || !database) return;
+    if (!isSchemaAware(connectionId)) return;
+    const dbType = connectionStore.getConfig(connectionId)?.db_type;
+    if (!database && !isSingleDatabase(dbType)) return;
     const key = cacheKey(connectionId, database);
     if (loadingSchemaOptions.value[key]) return;