Fix: add_sig_param: pass annotation to Parameter

[RensDimmendaal]

add_sig_param builds the injected Parameter without annotation=, storing the type only in f.__annotations__. This used to work because fastcore's old signature_ex merged __annotations__ back over an explicit __signature__, an undocumented side effect of its pre-3.10 backport.

The signature_ex rewrite in fastcore delegates to stock inspect.signature, which returns __signature__ verbatim. Injected params therefore arrive with no annotation and are ignored by the request machinery. Which explains the failing test in #916

Fix: pass annotation=typ when constructing the Parameter, putting the type in the signature itself.