Don't spam config poller cache miss retries

[ogtownsend]

NOPs reported excessive error log spam from processSourceChainConfigResults() when a source chain's GetSourceChainConfig RPC call fails persistently (e.g., due to an RPC misconfiguration like gas limit below intrinsic gas).

Root cause:

• When GetAllConfigsLegacy fetches source chain configs and receives some failure, processSourceChainConfigResults logs the error and skips that chain and it's never added to the config poller cache.
• In configPollerV2.GetOfframpSourceChainConfigs(), any chain not in the cache is treated as a "cache miss," which triggers an inline call to batchRefreshChainAndSourceConfigs.
• Since the failing chain never gets cached, every subsequent call triggers another full batch fetch, producing the same error logs every time.
• Two concurrent callers in the merkle root asyncObserver.sync() (ObserveOffRampNextSeqNums and ObserveLatestOnRampSeqNums) both hit this path on every tick, multiplying the log volume.

Fix:
pkg/reader/config_poller_v2.go:

• Added attemptedSourceChains map to chainCache to track which source chains have been included in at least one fetch attempt.
• GetOfframpSourceChainConfigs: A chain missing from the cache is only treated as a "cache miss" (triggering an inline fetch) if it has never been attempted. Chains that were previously attempted but failed are not re-fetched inline
• The background poller (refreshAllKnownChains) will still retry them on its regular schedule.
• batchRefreshChainAndSourceConfigs: After each fetch, all requested source chain selectors are recorded in attemptedSourceChains and sourceChainRefresh is always updated, regardless of whether individual chains succeeded.

pkg/chainaccessor/config_processors.go:

• Downgraded the "Failed to get source chain config from result" log from Errorw to Warnw.
• This is a per-chain partial failure that is retried by the background poller, Error level creates excessive noise

[github-actions]

Metric	ogt/dont-spam-retry-source-chain-config-fetch-retries	main
Coverage	70.1%	69.7%