Fix edge case for continue_final_message

[Rocketknight1]

The code for continue_final_message in apply_chat_template fails in the following edge case:

• The chat template trims trailing spaces from messages
• The chat template adds its own spacing after messages
• The unstripped content of the final message is identical to the stripped content of a previous message plus the template's built-in spacing

This causes the template to incorrectly identify the previous message as the final message and strip off a big chunk of the conversation after this point. This is a fairly uncommon edge case, but it's worth fixing nonetheless! cc @TK-21st

This PR handles things more elegantly without a try-except block, and also improves handling when assistant messages contain text/image blocks (which is rare right now, but will probably become more common in future)

Fixes #35433

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[Rocketknight1]

cc @Cyrilvallez @ArthurZucker for core maintainer review!