kvcache: Log batch size if we can't find a slot

In some cases, we can't find a cache slot when using sliding window
attention. It would be helpful in this (and other cases) to know what
the batch size is.

Bug #10127

kvcache/causal.go

@@ -239,7 +239,7 @@ func (c *Causal) findStartLoc() (int, error) {
 		}
 	}
 
-	return 0, fmt.Errorf("%w (length: %v)", ErrKvCacheFull, len(c.cells))
+	return 0, fmt.Errorf("%w (cache: %v batch: %v)", ErrKvCacheFull, len(c.cells), c.curBatchSize)
 }
 
 func (c *Causal) updateSlidingWindow() {