fix(plugin): rename header keys instead of overwriting values in modify-response

[Sean-Walker0]

Fixes #6510

Modifications

ModifyResponsePlugin applied the replaceHeaderKeys map with httpHeaders.replace(key, Collections.singletonList(value)). HttpHeaders.replace keeps the key and replaces the values, so a rule {"X-Old": "X-New"} against an upstream X-Old: abc returned X-Old: X-New instead of renaming the header to X-New: abc — contradicting the documented contract (key: oldHeaderKey, value: newHeaderKey) and the sibling RequestPlugin#replaceHeaderKey, which renames the key while preserving the values.

The fix renames the header key and carries the original values over, mirroring the request-side implementation:

replaceHeaderMap.forEach((key, value) -> {
    List<String> values = httpHeaders.get(key);
    if (Objects.nonNull(values)) {
        httpHeaders.addAll(value, values);
        httpHeaders.remove(key);
    }
});

Verifying this change

• New ModifyResponsePluginTest#testReplaceHeaderKeysRenamesKeyAndKeepsValues — fails on the pre-fix code (expected: <false> but was: <true>, the old key survived and the values were overwritten) and passes after the fix; it also asserts multiple header values are preserved under the new key.
• New ModifyResponsePluginTest#testReplaceHeaderKeysLeavesMissingSourceKeyUntouched — a configured source key absent from the response stays a no-op.
• ./mvnw -pl shenyu-plugin/shenyu-plugin-modify-response -am test -B — 13/13 module tests green, checkstyle clean.

Notes

• Behavior change (the bug fix itself): responses configured with replaceHeaderKeys now return the header under the new name with the original values, instead of the old name with the new name as the value. This matches the rule-handle javadoc and the request-plugin sibling.
• Side finding (not addressed here): addHeaders uses httpHeaders::add, which appends to existing values rather than replacing them; that is a separate semantic question for maintainers.
• Orthogonality: no open PR touches shenyu-plugin-modify-response (verified against the file lists of all 92 open PRs).

Make sure that:

• You have read the contribution guidelines.
• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true (module-scoped: shenyu-plugin/shenyu-plugin-modify-response with -am, tests + checkstyle green).