Fix log content duplication in ParseJsonNativeProcessor

[quzard]

This commit resolves an issue in ParseJsonNativeProcessor where the original log content was unintentionally retained in the "content" field due to improper state management.

The processor incorrectly maintained a class-level state indicating whether the "content" key was overwritten during JSON parsing. While this state should be reset for each new log, it was persistently kept across logs. Consequently, if a JSON log contained a "content" key, the processor would mark the state as overwritten and not drop the "content" field in subsequent logs, leading to duplicated content.

To address this, the state tracking the "content" key overwrite is moved from class level to local level, ensuring it is reset at the start of each log parsing operation.