Consider supporting template strings in `Template` specifications in Python 3.14+

[eb8680]

Python 3.14 added t-strings, which allow programmable reuse of f-string literals as templates (including those containing arbitrary expressions, which are not accessible to string.Formatter).

We could allow handlers.llm.template.Template docstrings to be valid instances of string.templatelib.Template in Python >=3.14, though there does not seem to be a backport of string.templatelib to <3.14 because of backwards-incompatible syntax changes.