Prevent caching of responses resolved via fallback

[Septrum101]

Effect of this fix

After the fix:

• When the primary DNS is unavailable:

  • The client can still receive responses from the optimistic cache.
  • If the background refresh eventually uses the fallback DNS, that response will not overwrite the cache.

• When the primary DNS recovers:

  • The next successful background refresh through the primary DNS will update the cache.
  • The primary DNS response will be written back into the cache.
  • Subsequent requests will return cached results from the primary DNS again.

Note

This fix changes one behavior:
Fallback DNS responses will no longer refresh the cache TTL during optimistic cache refresh.
This is usually expected, because fallback DNS is a failover path and should not continue to pollute the primary cache after the primary DNS has recovered.