[BUG] MockPlugin NPE when httpStatusCode is null in MockHandle (unboxing null Integer to HttpStatus.valueOf)

## Description
When `rewriteHandle.getReplace()` contains `{`, the code enters the placeholder-substitution branch: `PathMatchUtils.replaceAll(replace, regex.substring(regex.indexOf("{")), rewriteUri.substring(regex.indexOf("{") + 1))`. (1) Off-by-one: `regex.indexOf("{")` is the position of `{` in the regex, which in the URI corresponds to the first character of the path-variable value (regex and URI share the same prefix up to the `{`). The `+1` skips that first character. E.g. regex=`/http/findById/{id}` (`indexOf("{")`=15), URI=`/http/findById/123`, `substring(16)`=`23` instead of `123`. (2) `StringIndexOutOfBoundsException`: if `replace` contains `{` but `regex` does not, `regex.indexOf("{")` returns -1, `regex.substring(-1)` throws. Neither failure mode is covered by tests.

## Location
- `shenyu-plugin/shenyu-plugin-rewrite/src/main/java/org/apache/shenyu/plugin/rewrite/RewritePlugin.java:77-79`

## Impact
(1) Silent wrong upstream path for every request using path-variable rewrite templates — upstream receives a truncated path segment, likely 404 or wrong resource. (2) 500 on every request when replace template has `{` but regex is a plain regex without `{`.

## Suggested fix
Change `rewriteUri.substring(rewriteHandle.getRegex().indexOf("{") + 1)` to `rewriteUri.substring(rewriteHandle.getRegex().indexOf("{"))` (remove `+1`). Guard the entire branch with a check that `regex.indexOf("{") >= 0`.

## Related existing
None — distinct from #6805 (RewriteHandle equals/hashCode omit percentage) which is about the DTO's equals/hashCode, not the rewrite logic.

Start in shenyu-plugin/shenyu-plugin-rewrite/src/main/java/org/apache/shenyu/plugin/rewrite/RewritePlugin.java at lines 77-79 and trace the placeholder-substitution branch. Verify the path-variable case preserves the full value and that a replacement containing `{` without a regex placeholder no longer fails; add regression coverage for both cases and run the relevant rewrite tests.

Written by the indexing model from the issue text.