fix: update adding_new_models notebook to use current ChatTemplate API

[octo-patch]

Fixes deprecated API usage in adding_new_models.ipynb notebook. Replaces TransformersChat with ChatTemplate subclass and removes indent_roles() calls.

[ErenAta16]

This notebook fix is identical to #1450 (also open, same author, confirmed byte-identical diff on adding_new_models.ipynb), but this branch is based on an old main and its diff against current main pulls in a lot of unrelated churn: it would delete guidance/_uri_validation.py and revert tests/unit/test_bytes_from.py/test_uri_validation.py by 89/137 lines, plus revert several .github/workflows/*.yml changes and other file updates that have landed since this branch was cut. That _uri_validation.py/test_bytes_from.py piece in particular looks like it would undo #1464 (the redirect-revalidation SSRF fix I reviewed earlier), which is already merged-or-mergeable — not something anyone here intends, just a side effect of the branch being stale.

#1450 has the identical notebook fix on a current base with none of that collateral diff, so recommending that one instead — this one is probably worth closing or rebasing rather than merging as-is.