* refactor: extract resolve_coalesced, rewrite tests against real code
Extract Disposition enum, acquire_inflight(), and resolve_coalesced()
from handle_query so coalescing logic is independently testable. Rewrite
integration tests to call resolve_coalesced directly with mock futures
instead of fighting the iterative resolver's NS chain. All 12 coalescing
tests now exercise production code paths, not tokio primitives.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
* fix: SERVFAIL echoes question section, preserve error messages
resolve_coalesced now takes &DnsPacket instead of query_id so SERVFAIL
responses use response_from (echoing question section per RFC). Error
messages preserved via Option<String> return for upstream error logging.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>