Fix UDP DNS hang during network switching #3653
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We mistakenly believed that `libresolv`'s `search` function worked correctly in NetworkExtension, but it seems only `getaddrinfo` does. This commit changes the behavior of the `local` DNS server in NetworkExtension to prefer DHCP, falling back to `getaddrinfo` if DHCP servers are unavailable. It's worth noting that `prefer_go` does not disable DHCP since it respects Dial Fields, but `getaddrinfo` does the opposite. The new behavior only applies to NetworkExtension, not to all scenarios (primarily command-line binaries) as it did previously. In addition, this commit also improves the DHCP DNS server to use the same robust query logic as `local`.
We do not have the `com.apple.developer.networking.multicast` entitlement and are unable to obtain it for non-technical reasons.
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UDPTransport.open()held the mutex whileDialContextcould block during Wi‑Fi changes, soClose()waited on the same lock and timed out. Move UDP dialing outside the lock and explicitly close the active UDP connection inClose(), so shutdown completes even when dialing stalls