fix: zero-initialize prefetch query options in cache timer callbacks

[PikuZheng]

Motivation

• Prevent timer-triggered prefetch from reusing stale stack data in dns_server_query_option, which can cause accumulated/contaminated request options and unexplained memory growth when the cache timer fires.

Description

• Zero-initialize the stack struct dns_server_query_option in the cache prefetch callbacks by changing to struct dns_server_query_option server_query_option = {0}; in _dns_server_prefetch_domain and _dns_server_prefetch_expired_domain in src/dns_server/prefetch.c so each timer execution starts from a clean state.

Testing

• Built the project with make -C src -j4, which completed successfully (compile warnings unrelated to this change were present but the build succeeded).

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Codex Task