refactor: Remove unreachable dead code branches

- errors.isRetryable: Convert final case to default (all ErrorType values covered)
- scheduler.SelectInterval: Remove bounds checks after mathematical computation
  that guarantees target ∈ [min, max]

Both functions now at 100% coverage.

internal/errors/errors.go

@@ -116,14 +116,13 @@ func isRetryable(errorType ErrorType, err error) bool {
 		return true
 	case ErrorTypeAuth, ErrorTypeValidation, ErrorTypeNotFound:
 		return false
-	case ErrorTypeInternal, ErrorTypeAPI:
+	default: // ErrorTypeInternal, ErrorTypeAPI
 		// Check the underlying error
 		if err != nil {
 			return !errors.Is(err, ErrInvalidInput) && !errors.Is(err, ErrForbidden)
 		}
 		return true
 	}
-	return false
 }
 
 // Helper functions

internal/monitoring/scheduler.go

@@ -305,15 +305,9 @@ func (a *adaptiveIntervalSelector) SelectInterval(req IntervalRequest) time.Dura
 
 	score := clampFloat(req.StalenessScore, 0, 1)
 	span := float64(max - min)
+	// target is mathematically in [min, max] since score ∈ [0,1] and span >= 0
 	target := time.Duration(float64(min) + span*(1-score))
 
-	if target < min {
-		target = min
-	}
-	if target > max {
-		target = max
-	}
-
 	if req.ErrorCount > 0 {
 		penalty := 1 + a.errorPenalty*float64(req.ErrorCount)
 		if penalty > 0 {