server : add /apply-template endpoint for additional use cases of Minja functionality

[pnb]

This PR adds an endpoint to examples/server, /apply-template, which will apply the model's chat template to the given messages, like for chat completion, but then will simply return the formatted prompt rather than running inference.

Sometimes I modify prompts after formatting them for chat interactions, especially for cases where the goal is to insert something into the beginning of the model's response. For example, the classic "Sure!" insertion to reduce refusals, or more often some partially-constructed solutions that I would like the model to finish.

Previously I have implemented chat templates for each model myself, but with the addition of some Jinja support via Minja it is very tempting to let the server do this for users instead, as it is more likely to be correct and offer good coverage of prompt templates.

This PR also adds a CI test, which I did using the Command-R template to provide some variety versus the closely related test_chat_template unit test (side note: I did not find __verbose very useful for prompt formatting use cases because it includes BOS, hence the new endpoint).