feat(config): add temporary logging for failed reloads

- Add tmpLogBuf and tmpLog fields to capture config loading logs
  - Flush temporary logs only when reload succeeds
  - Extract NewLogger function for creating custom loggers
  - Update State interface to include FlushTmpLog method

internal/config/types/state.go

@@ -30,6 +30,8 @@ type State interface {
 	IterProviders() iter.Seq2[string, types.RouteProvider]
 	NumProviders() int
 	StartProviders() error
+
+	FlushTmpLog()
 }
 
 // could be nil