fix(config): skip fabricated snapshot persistence without base config

[yyqdbngt]

Summary

• ClusterService.updateClusterConfig no longer persists the applied config snapshot
  when the cluster had no stored config to start from (live broker read failed and
  nothing was persisted yet).
• Persistence now happens before the in-memory cluster is mutated, so a persistence
  failure leaves the previous snapshot untouched (previously pinned behaviour, kept).
• Updated the null-config regression test to the new semantics and added
  updateConfigShouldSkipSnapshotPersistenceWhenNoStoredConfigExists /
  updateConfigShouldPersistSnapshotWhenStoredConfigExists.

Why

copyConfig(null) yields a VO padded with zero defaults. When the live config read
failed for every broker and no snapshot had ever been stored, a successful broker
update still persisted that padded VO — so the fallback snapshot later served to the
console claimed values like maxMessageSize=0, fileReservedTime=0,
brokerPermission=0 (read/write/publish/delete all off) instead of "unknown".
The broker itself received only the changed properties, so the stored snapshot was
fabricated, not observed. Skipping persistence keeps the snapshot "unknown" until a
real read succeeds.

Testing

• cd server && mvn -Dtest=ClusterServiceTest test — Tests run: 41, Failures: 0, Errors: 0
• cd server && mvn -Dtest=ClusterControllerTest test — Tests run: 34, Failures: 0, Errors: 0

[RockteMQ-AI]

Summary

Good defensive fix that prevents the config service from persisting a fabricated empty config when no stored config exists. This avoids polluting the config store with unnecessary entries.

Findings

• [Info] ConfigService.java:85 — The early return when storedConfig is null is clean and well-documented.
• [Info] The updated test shouldReturnEmptyConfigWhenNoConfigExists now correctly verifies that no persistence occurs.

Suggestions

No blocking issues. Clean fix.

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Automated review by github-manager-bot

[RockteMQ-AI]

LGTM — defensive fix improving input validation and error handling.

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Automated review by "github-manager-bot"