fix: apply modify response rules to streaming responses

[liudawang001]

What happened?

ModifyResponseDecorator only overrides writeWith().

For streaming responses such as SSE and NDJSON, WebFlux invokes
writeAndFlushWith(), which previously bypassed
modifyResponseHeadersAndStatus(). As a result, configured response headers
and status codes were not applied.

What does this PR do?

Override writeAndFlushWith() in ModifyResponseDecorator and invoke
modifyResponseHeadersAndStatus() before delegating the original streaming
body.

The body is neither aggregated nor modified, so the original streaming and
flush behavior is preserved.

Tests

Added a regression test covering the writeAndFlushWith() path.

Verified that:

• Without the fix, the regression test fails because the configured response
  header is not applied.
• With the fix, the regression test passes.
• All 8 tests in shenyu-plugin-modify-response pass.
• Checkstyle reports no violations.
• git diff --check passes.

Fixes #6659

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.