Add unit tests and context-based watcher cancellation for loopcheck removal

[Copilot]

No *_test.go files existed to validate the removal of loopcheck and the switch to an always-on fsnotify watcher.

Tests added (cblogger_test.go)

• TestNewCBLOGCONFIG_Defaults — confirms LOOPCHECK field is absent from the struct (would be a compile error if reintroduced)
• TestGetConfigInfos_WithoutLoopcheck — config YAML without loopcheck key parses correctly
• TestGetConfigInfos_OldConfigWithLoopcheck — old YAML with loopcheck: true loads without error; unknown field is silently ignored by yaml.v3
• TestDynamicLevelChange — end-to-end: writes a config file, starts the logger, overwrites loglevel, and asserts the watcher picks up the change within 3 s

Production fixes (surfaced during testing)

Goroutine leak / nil-pointer panic — levelSetupWatcher ran forever with no shutdown path; resetting globals in tests caused a panic when the dangling goroutine called SetLevel on a nil logger.

• setup() now creates a context.WithCancel and stores watcherCancel
• levelSetupWatcher accepts ctx context.Context and exits on ctx.Done()
• reloadConfig() helper extracted to remove the duplicated GetConfigInfos + SetLevel pair across Rename/Remove and Write/Create branches

// before
go levelSetupWatcher(loggerName, configFilePath)

// after
ctx, cancel := context.WithCancel(context.Background())
watcherCancel = cancel
go levelSetupWatcher(ctx, loggerName, configFilePath)

Test run results

=== RUN   TestNewCBLOGCONFIG_Defaults       --- PASS (0.00s)
=== RUN   TestGetConfigInfos_WithoutLoopcheck --- PASS (0.00s)
=== RUN   TestGetConfigInfos_OldConfigWithLoopcheck --- PASS (0.00s)
=== RUN   TestDynamicLevelChange             --- PASS (0.30s)
ok  github.com/cloud-barista/cb-log  0.306s

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