🐛 fix(query-editor): 修复多数据源大查询限流失效

- SQL限流：抽取查询自动限流工具，修复 SELECT 判断大小写不一致导致限制未生效
- 方言适配：按 Oracle/Dameng、SQL Server、MySQL/PostgreSQL 等方言分别注入行数限制
- 自定义驱动：支持 custom 连接根据 driver 解析 Oracle、PostgreSQL、SQL Server 等方言
- MongoDB修复：修正 db.collection.find() 解析边界，并对 find/只读 aggregate 下推 limit
- Oracle优化：DSN 增加 PREFETCH_ROWS 和 LOB FETCH 参数，减少大结果集拉取开销
- 测试覆盖：补充 SQL 方言矩阵、MongoDB 限流和 Oracle DSN 参数测试
Refs #424

frontend/src/utils/mongodb.test.ts

@@ -1,6 +1,8 @@
 import { describe, expect, it } from 'vitest';
 
-import { convertMongoShellToJsonCommand } from './mongodb';
+import { applyMongoQueryAutoLimit, convertMongoShellToJsonCommand } from './mongodb';
+
+const parseCommand = (command: string | undefined) => JSON.parse(command || '{}');
 
 describe('convertMongoShellToJsonCommand', () => {
   it('converts show dbs shell shortcut to listDatabases command', () => {
@@ -16,4 +18,105 @@ describe('convertMongoShellToJsonCommand', () => {
       command: JSON.stringify({ listCollections: 1, filter: {}, nameOnly: true }),
     });
   });
+
+  it('converts find shell commands without adding implicit limit', () => {
+    const result = convertMongoShellToJsonCommand('db.users.find({ active: true })');
+
+    expect(result.recognized).toBe(true);
+    expect(parseCommand(result.command)).toEqual({
+      find: 'users',
+      filter: { active: true },
+    });
+  });
+
+  it('keeps explicit find limit values from shell commands', () => {
+    const result = convertMongoShellToJsonCommand('db.users.find({}).limit(10)');
+
+    expect(parseCommand(result.command)).toEqual({
+      find: 'users',
+      filter: {},
+      limit: 10,
+    });
+  });
+
+  it('keeps explicit zero limit values from shell commands', () => {
+    const result = convertMongoShellToJsonCommand('db.users.find({}).limit(0)');
+
+    expect(parseCommand(result.command)).toEqual({
+      find: 'users',
+      filter: {},
+      limit: 0,
+    });
+  });
+});
+
+describe('applyMongoQueryAutoLimit', () => {
+  it('adds limit to raw Mongo find commands', () => {
+    const result = applyMongoQueryAutoLimit('{"find":"users","filter":{}}', 500);
+
+    expect(result.applied).toBe(true);
+    expect(parseCommand(result.command)).toEqual({
+      find: 'users',
+      filter: {},
+      limit: 500,
+    });
+  });
+
+  it('adds limit after shell find conversion', () => {
+    const shell = convertMongoShellToJsonCommand('db.users.find({ active: true })');
+    const result = applyMongoQueryAutoLimit(shell.command || '', 500);
+
+    expect(result.applied).toBe(true);
+    expect(parseCommand(result.command)).toEqual({
+      find: 'users',
+      filter: { active: true },
+      limit: 500,
+    });
+  });
+
+  it('does not replace explicit find limits', () => {
+    const result = applyMongoQueryAutoLimit('{"find":"users","filter":{},"limit":10}', 500);
+
+    expect(result.applied).toBe(false);
+    expect(parseCommand(result.command)).toEqual({
+      find: 'users',
+      filter: {},
+      limit: 10,
+    });
+  });
+
+  it('adds $limit to read-only aggregate pipelines', () => {
+    const result = applyMongoQueryAutoLimit('{"aggregate":"users","pipeline":[{"$match":{"active":true}}],"cursor":{}}', 500);
+
+    expect(result.applied).toBe(true);
+    expect(parseCommand(result.command)).toEqual({
+      aggregate: 'users',
+      pipeline: [
+        { $match: { active: true } },
+        { $limit: 500 },
+      ],
+      cursor: {},
+    });
+  });
+
+  it('does not add another aggregate $limit', () => {
+    const command = '{"aggregate":"users","pipeline":[{"$limit":10}],"cursor":{}}';
+    const result = applyMongoQueryAutoLimit(command, 500);
+
+    expect(result.applied).toBe(false);
+    expect(result.command).toBe(command);
+  });
+
+  it('does not alter aggregate write pipelines', () => {
+    const command = '{"aggregate":"users","pipeline":[{"$match":{}},{"$out":"tmp_users"}],"cursor":{}}';
+    const result = applyMongoQueryAutoLimit(command, 500);
+
+    expect(result.applied).toBe(false);
+    expect(result.command).toBe(command);
+  });
+
+  it('does not limit non-read or invalid commands', () => {
+    expect(applyMongoQueryAutoLimit('{"count":"users","query":{}}', 500).applied).toBe(false);
+    expect(applyMongoQueryAutoLimit('db.users.find({})', 500).applied).toBe(false);
+  });
 });