dns: preserve failures in dual-family resolution

[yuchen-db]

In AUTO and V4_PREFERRED mode, a no-records response from one address family can leave the combined result marked Completed even when the other lookup fails. STRICT_DNS consumers can then replace their existing hosts with an empty list.

Track lookup failures through completion and return Failure when no addresses were obtained. Successful fallback addresses and successful empty lookups remain supported. Intentionally filtered address families do not count as DNS failures.

Testing

bazel test --config=clang -c fastbuild //test/extensions/network/dns_resolver/cares:dns_impl_test passed all 148 tests, including IPv4 and IPv6, in the official build image. Format and spelling checks passed.
Docs Changes: Documented c-ares dual-family failure handling.

AI assistance: Codex assisted with the code and tests

Fixes #38752

[repokitteh-read-only]

Hi @yuchen-db, welcome and thank you for your contribution.

We will try to review your Pull Request as quickly as possible.

In the meantime, please take a look at the contribution guidelines if you have not done so already.

🐱

Caused by: #47494 was opened by yuchen-db.

see: more, trace.

[agrawroh]

/assign @yanavlasov