TopicConfig.decode mutates state or throws for malformed config

TopicConfig.decode assigns topicName before parsing numeric and enum fields. A malformed serialized config can throw IllegalArgumentException after partially mutating the target object. Expected behavior: return false and leave the existing object unchanged when mandatory fields cannot be parsed.

Start at TopicConfig.decode and trace how topicName, numeric fields, and enum fields are parsed and assigned. Verify malformed mandatory fields return false without changing the existing object, and confirm valid configurations still decode successfully.

Written by the indexing model from the issue text.