[BUG] Context path plugin rewrites realUrl without checking path prefix

### 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

`ContextPathPlugin` builds `realUrl` by cutting the request path at `contextPath.length()`:

```java
String realURI = "";
String contextPath = handle.getContextPath();
if (StringUtils.isNoneBlank(contextPath)) {
realURI = context.getPath().substring(contextPath.length());
attributes.put(Constants.CONTEXT_PATH, contextPath);
}
```

There is no check that `context.getPath()` actually starts with the configured `contextPath`, or that the configured context path is not longer than the request path.

This produces two failure modes:

- If `contextPath` is longer than the request path, `substring(contextPath.length())` throws `StringIndexOutOfBoundsException` and the request fails.
- If `contextPath` has the same/shorter length but is not a prefix, the plugin silently cuts the wrong characters and sets an incorrect `realUrl`, causing downstream routing to the wrong path.

For example, with request path `/api/order` and configured `contextPath=/foo`, the plugin sets `realUrl=/order` even though `/foo` was never a prefix of the request path.

### Expected behavior

`ContextPathPlugin` should only strip `contextPath` when the request path starts with that context path. If the configured context path does not match the request path, the plugin should skip rewriting or return a clear rule/configuration error instead of throwing or silently producing an incorrect `realUrl`.

### How to reproduce

1. Enable the context-path plugin.
2. Create a rule with `contextPath` that does not match the beginning of the incoming path, for example `contextPath=/foo`.
3. Send a request to `/api/order`.
4. The plugin computes `context.getPath().substring(contextPath.length())` and produces `/order`, incorrectly removing the first four characters even though `/foo` was not present.
5. If `contextPath` is longer than the request path, the same code throws `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 the ContextPathPlugin code containing the realURI substring call and inspect how context.getPath(), contextPath, and realUrl are used. Add coverage for a nonmatching and longer context path, then verify that rewriting occurs only for a matching prefix and that the existing plugin tests pass.

Written by the indexing model from the issue text.