[fix](dns-cache) evict hostnames that fail to resolve repeatedly

[zhaorongsheng]

Proposed changes

Issue Number: close #63358

DNSCache currently never evicts an entry once it has been inserted. When a backend host is permanently dropped from the cluster and its DNS record is removed, every other BE in the cluster keeps:

1. logging failed to get ip from host: every 60 s, indefinitely (from DNSCache::_refresh_cache -> hostname_to_ipv4);
2. handing the stale cached IP back to brpc through BrpcClientCache / ClientCache, which then keeps emitting Fail to wait EPOLLOUT ... Connection timed out at the brpc socket layer.

This PR adds a simple consecutive-failure counter to DNSCache. When the counter reaches a configurable threshold, the entry is removed from the cache so that callers no longer get a stale IP and the refresh thread stops logging about
it. WARNING logs for the same host are also throttled to avoid flooding be.WARNING.

Configs introduced

Name: dns_cache_max_consecutive_failures
Type: mInt32
Default: 30
Behavior: Evict a hostname after this many consecutive resolution failures. At the default 60 s refresh interval, that means ~30 minutes of grace. Set <= 0 to disable eviction (legacy behavior).
────────────────────────────────────────
Name: dns_cache_log_every_n_failures
Type: mInt32
Default: 60
Behavior: Throttle the Failed to resolve ... use cached ip warning to once per N failures per hostname. Set <= 1 to log every failure (legacy behavior).

Both are mutable so operators can tune without restarting BE.

Backward compatibility

Setting dns_cache_max_consecutive_failures = 0 and dns_cache_log_every_n_failures = 1 reproduces exactly the pre-PR behavior. Successful resolution clears the failure counter, so transient DNS hiccups don't accumulate across hours. No
public API or wire format changes.

Further comments

• I deliberately did not touch the FE-side DNSCache.java. If the same fix is wanted on FE, happy to send a follow-up PR.
• The eviction threshold default (30) is conservative; please push back if you'd prefer a smaller / larger default. Operators with very flaky DNS can lower it via the mutable config without redeploying.

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Checklist

• I have read the Contributing document.
• I have created an issue ([Bug] DNSCache never evicts unresolvable hostnames after a BE is dropped, causing be.WARNING flood and persistent brpc EPOLLOUT timeout #63358) on (or commented on) the related issue.
• I have added unit tests for my change.
• All new and existing tests passed (verified locally → fails to build on macOS due to unrelated contrib/openblas issue; rely on CI).
• My change requires a change to the documentation. — No (config doc auto-generated)

[hello-stephen]

Thank you for your contribution to Apache Doris.
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[zhaorongsheng]

/review

[zhaorongsheng]

/review