kvcache: Log contents of cache when unable to find a slot

There is a bug when using sliding window attention where we run
out of KV cache slots. This is likely due to not correctly removing
all of the entries as they slide out of range. This adds additional
logging when this occurs to track down the source.

Bug #10127

kvcache/causal.go

@@ -214,6 +214,7 @@ func (c *Causal) StartForward(ctx ml.Context, batch input.Batch, reserve bool) e
 			c.curLoc, err = c.findStartLoc()
 		}
 		if err != nil {
+			slog.Warn("unable to find a kv cache slot", "cache", c)
 			return err
 		}