First things the model you extend Mage_Catalog_Model_Catalog_Product I think this is not the right name for the model ??
did you mean ? Mage_Catalog_Model_Product
Second with your code you extended the model but you didn't rewrite the model. So you have to change the model the in block files in the core pool which is not good.
So what u need to do here is to specify rewrite for the model in your config.xml
<models>
....
<catalog>
<rewrite>
<product>Beckin_TextShortner_Model_Catalog_Product</product>
</rewrite>
</catalog>
....
</models>
Then you will be able to use the method without issues