Avoid reusing released APIServer response buffers

[MrlixiangWE]

Fixes #272.

PubHandler and KillHandler previously reused static reference-counted ByteBuf instances for validation-error responses. Once Netty released the first response, the same validation path could receive content with refCnt == 0.

This change keeps the error text as static immutable strings and creates an independently owned UTF-8 buffer for every response. The regression tests release the first response before exercising the same path again and cover all five affected validation responses.

Tests:

• ./mvnw -q -pl bifromq-apiserver -Dtest=PubHandlerTest,KillHandlerTest test
• ./mvnw -q -pl bifromq-apiserver test
• ./mvnw -q -pl bifromq-apiserver checkstyle:check
• ./mvnw -q test