`Template` constructor should validate docstring formatting

[eb8680]

When constructing Templates, we should check that the variables in the docstring match the Template signature. The string should be a valid format string that refers to valid lexical variables in the Template body's lexical scope.

For example, the following should always be disallowed when author is undefined:

@Template.define
def write_poem(topic: str) -> str:
    """Write a poem about {topic} by {author}"""
    raise NotHandled

When author is a lexical variable in an enclosing scope, we could allow it to be referenced, or we could be strict and require Templates to refer only to arguments:

def poet(author: str) -> Template[[str], str]:
    @Template.define
    def write_poem(topic: str) -> str:
        """Write a poem about {topic} by {author}"""
        raise NotHandled
    return write_poem

These checks could be implemented by attempting to parse the template according to the official format string mini-language specification, and by attempting to substitute dummy values into variable names via .format(**self.__signature__.bind(*dummy_args, **dummy_kwargs).arguments)

Note: Python 3.14 adds a new module string.templatelib with a richer backwards-incompatible template string semantics; if we choose to support that in our prompt templates, which is out of scope for this issue, we should do similar validation.

[eb8680]

This will be especially important for #430