[BUG] Rewrite plugin can throw when replace uses placeholders but regex has no placeholder

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/shenyu/issues) and found no similar issues.

### Apache ShenYu Component

shenyu-plugin

### What happened

`RewritePlugin` switches to placeholder replacement whenever `replace` contains `{`:

```java
rewriteUri = rewriteHandle.getReplace().contains("{")
? PathMatchUtils.replaceAll(rewriteHandle.getReplace(), rewriteHandle.getRegex().substring(rewriteHandle.getRegex().indexOf("{")),
rewriteUri.substring(rewriteHandle.getRegex().indexOf("{") + 1))
: rewriteUri.replaceAll(rewriteHandle.getRegex(), rewriteHandle.getReplace());
```

This assumes `regex` also contains `{`. If the rule's `replace` contains a placeholder but `regex` does not, `rewriteHandle.getRegex().indexOf("{")` returns `-1`, and `regex.substring(-1)` throws `StringIndexOutOfBoundsException`.

A misconfigured rewrite rule should not crash request processing with an unchecked substring error.

### Expected behavior

The rewrite plugin should validate placeholder mode consistently. If `replace` contains placeholders, `regex` should also be required to contain the matching placeholder pattern before substring operations are performed. Invalid rewrite config should be rejected when the rule is saved/synced or skipped with a clear error instead of failing the request.

### How to reproduce

1. Enable the rewrite plugin.
2. Create a rewrite rule with a `replace` value containing `{...}` but a `regex` value without `{`, for example:

```json
{
"regex": "/api/(.*)",
"replace": "/v1/{path}"
}
```

3. Send a request matching the rule.
4. `RewritePlugin` enters placeholder mode because `replace` contains `{`, then calls `regex.substring(regex.indexOf("{"))` with `-1`, causing `StringIndexOutOfBoundsException`.

### Debug logs

_No response_

### Environment

Current `master` branch.

### Are you willing to submit a PR?

- [ ] Yes I am willing to submit a PR!

Start at RewritePlugin and trace the placeholder branch that calls PathMatchUtils.replaceAll; inspect how regex and replace values are validated or processed. Reproduce the example rule with a placeholder only in replace, then verify that invalid configuration no longer causes an unchecked substring error and instead follows the project's chosen clear-error or skip behavior.

Written by the indexing model from the issue text.