[test-improver] Improve tests for logger common package

[github-actions]

Test Improvements: internal/logger/common_test.go

File Analyzed

• Test File: internal/logger/common_test.go
• Package: internal/logger
• Lines of Code: ~696 → ~770 (74 lines added)

Improvements Made

1. Better Testify Patterns — Replace t.Errorf/t.Fatal with Idiomatic Assertions

Problem: Several tests used t.Errorf("should not be called") inside callback closures and t.Fatal inside if err == nil guards instead of proper testify patterns.

• ✅ TestInitLogFile_InvalidDirectory — replaced if err == nil { file.Close(); t.Fatal(...) } with require.Error(err, ...) after closing any file handle
• ✅ TestInitLogFile_EmptyFileName — same pattern; added require := require.New(t) so the test uses testify require.Error consistently
• ✅ TestInitLogger_FileLogger — replaced t.Errorf("Error handler should not be called") inside callback with a errorHandlerCalled boolean flag + assert.False(errorHandlerCalled, ...)
• ✅ TestInitLogger_FileLoggerFallback — replaced t.Errorf in setup callback with setupHandlerCalled flag + assert.False
• ✅ TestInitLogger_JSONLLogger — same improvement as FileLogger
• ✅ TestInitLogger_JSONLLoggerError — same improvement as FileLoggerFallback
• ✅ TestInitLogger_MarkdownLogger — same improvement as FileLogger
• ✅ TestInitLogger_MarkdownLoggerFallback — same improvement as FileLoggerFallback
• ✅ TestInitLogger_SetupError — replaced t.Errorf in error-handler callback with flag + assert.False

The boolean flag pattern makes the intent explicit alongside the other post-call assertions and avoids mixing raw t.Errorf with testify assertions in the same test.

2. Increased Coverage — Direct Tests for formatLogLine

Problem: The formatLogLine function in common.go was exercised only indirectly through FileLogger.Log() and ServerFileLogger, but had no direct unit tests. The function is responsible for the canonical log line format used across all file-based loggers.

• ✅ Added TestFormatLogLine with subtests covering:
  • All four log levels (INFO, WARN, ERROR, DEBUG) appear in brackets
  • Category appears in its own bracket
  • Format-string interpolation with args (count=%d name=%s)
  • Full bracket structure regex: ^\[\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}Z\] \[INFO\] \[auth\] event occurred$
  • Timestamp is RFC3339 UTC and falls within the test's time window
  • Format string used as-is when no args provided
  • Empty category produces [] bracket pair

3. Cleaner & More Stable Tests

• ✅ Added "strings" and "time" imports for the new TestFormatLogLine test
• ✅ Consistent testify usage — no more mixing of t.Errorf with assert/require in the same tests
• ✅ Better resource cleanup — files are closed before require.Error stops the test

Why These Changes?

internal/logger/common_test.go tests the core initialization primitives (closeLogFile, initLogFile, initLogger) used by all logger types in the package. The formatLogLine function — which defines the log line format for all operational logs — was entirely untested directly. The t.Errorf callback pattern, while valid Go, was inconsistent with the testify style used throughout the rest of the file.

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