feat: only cache final target record when dns sever response

[AlinsRan]

Background

The current DNS resolving and caching logic operates at the RR (Resource Record) level.
Each record in a CNAME chain (CNAME / A / AAAA) is cached independently with its own TTL.

In real-world deployments, this behavior can lead to inconsistencies:

• An intermediate CNAME record may expire or become unavailable
• While the final A / AAAA record is still present in cache
• The resolver may continue using the cached final address
• This breaks the expected consistency of the DNS resolution chain

For gateway and traffic-routing components, this behavior can be undesirable.

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Practical Limitation of Intermediate Records

In addition to TTL consistency, there is a practical operational constraint:

Intermediate CNAME nodes are not always globally or persistently resolvable.

In multi-region, multi-cloud, or third-party DNS environments (e.g. CDN or cloud-internal domains):

• Intermediate domains may only be resolvable in specific regions or networks
• Re-querying intermediate CNAME records may fail depending on location

This can cause traffic to be routed based on a DNS chain that is no longer valid from the resolver’s perspective.

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Example

run.api7.ai.            10  IN CNAME   one.cloudfront.net.
one.cloudfront.net.     3   IN CNAME   two.cloudfront.net.
two.cloudfront.net.     10  IN A       18.155.68.66

Possible issue:

one.cloudfront.net expires or becomes unreachable in a given region

What’s Changed

This PR introduces an optional configuration flag:

finalCacheOnly: true

When finalCacheOnly is enabled:

• The resolver only caches the final A / AAAA record

• The cache TTL is calculated as the minimum TTL across the entire CNAME chain

• Intermediate CNAME records are not cached or reused independently

[CLAassistant]

All committers have signed the CLA.

[AlinsRan]

This domain name is no longer working.

[AlinsRan]

ref:

• chore: run test ci #8
• https://github.com/api7/lua-resty-dns-client/actions/runs/20611757545/job/59197621646?pr=8#step:6:1275

CI failure is due to some domain names no longer working, unrelated to the modification of this PR.
This PR has been fixed and temporarily annotated for some complex scenarios such as SRV+CNAME. Waiting for further repairs.