fix(ai): preserve headers on transform fallback

[dengliming]

Summary

• keep original response headers when the transformed response has no valid body
• apply AI-provided headers only when the transformed body is accepted
• add regression coverage for the fallback path

Testing

• mvn -q -pl shenyu-plugin/shenyu-plugin-ai/shenyu-plugin-ai-response-transformer -am -DskipTests=false -Dcheckstyle.skip=false -Dtest=AiResponseTransformerPluginTest -DfailIfNoTests=false test

Fixes #6756

[Aias00]

Approved as PMC (Aias00). Reviewed the diff — small, well-scoped fix with regression tests; mergeable.