fix(js/generate): handle when finishReason is length

[assafshemesh]

Currently when the model returns finishReason === 'length' the flow fails on schema validation (as output will probably be corrupted), hence hiding the real fail reason.
This fix will raise error properly on that case

Checklist (if applicable):

• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)