[Studio][Bug] Studio user search does not escape SQL LIKE wildcards

## Problem

`AuthService.listUsers` passes the raw username search into MyBatis-Plus `like("username", normalizedSearch)` without escaping SQL LIKE wildcards. A search for `admin_` matches `adminX`, `admin1`, etc.; a search for `admin%` matches any suffix.

Instance, credential, and operation-audit search paths already escape `\`, `%`, and `_` (see #4192–#4194). The studio user search path does not.

## Evidence

```java
String normalizedSearch = search == null ? "" : search.trim();
// ...
.like(!normalizedSearch.isEmpty(), "username", normalizedSearch)
```

`AuthService.java` `listUsers`.

## Expected behavior

Username search matches the literal search string. `_` and `%` in the query are not treated as wildcards.

## Scope

`AuthService.listUsers` + focused unit regression test. No API change.

Start in AuthService.java at listUsers and compare its username search with the escaping used by the instance, credential, and operation-audit search paths referenced in #4192–#4194. Add a focused unit regression test showing that literal _ and % characters do not act as SQL LIKE wildcards, then verify the search matches the literal username string.

Written by the indexing model from the issue text.