fix: update adding_new_models tutorial to use current API

[octo-patch]

Fixes #1402

Problem

The adding_new_models.ipynb tutorial uses the outdated API where custom model support was implemented by subclassing TransformersChat. This API no longer exists in v0.3.x.

The old approach:

class OrcaChat(models.transformers.TransformersChat):
    def get_role_start(self, role_name, **kwargs): ...
    def get_role_end(self, role_name=None): ...

orca = OrcaChat('pankajmathur/orca_mini_3b', ...)

Solution

Update the notebook to use the current ChatTemplate API:

• Replace OrcaChat(TransformersChat) with OrcaChatTemplate(ChatTemplate)
• Use models.Transformers(..., chat_template=OrcaChatTemplate) to load with a custom template
• Import UnsupportedRoleException for proper error handling on unsupported roles
• Remove stale cell outputs and update explanations to reflect the new architecture

Testing

The updated code is self-contained and follows the same pattern used in the existing guidance documentation and codebase (e.g., guidance/chat.py).