test: add comprehensive tests to improve coverage
- observability: added tests for LogLevel.String(), Warn(), Warnf(), Infof(),
Debugf(), Error(), WithFields(), and concurrent access patterns
- file: added tests for Reopen(), Close(), empty/whitespace paths,
validateFilePath allowed/rejected paths, concurrent writes, Flush(),
and marshal errors
- config: added tests for TimeWindowConfig.GetDuration(),
CorrelationConfig getters, validation scenarios (no inputs, no outputs,
duplicate sockets, ClickHouse validation), and LogConfig.GetLevel()
Coverage improvements:
- observability: 57.7% → 79.5%
- file: 68.6% → 78.6%
- config: 69.8% → 97.7%
- total: 68.6% → 74.4%
Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>