fix: don't offer unwrap result/option return type assist in trait impls

[Albab-Hasan]

fixes #13560

the unwrap_result_return_type and unwrap_option_return_type assists were incorrectly offered inside trait impl methods. applying the assist removes the return type breaking the trait contract and producing a compile error.

this fix disables both assists when the function is a trait impl method, checked via implemented_trait. this approach was chosen over showing the assist with a warning, since silently producing uncompilable code is wrong behavior.

added a test to cover this case.

[ChayimFriedman2]

Assists aren't required to produce compilable code, and this can be useful even in trait methods (for example, if you want to change the trait signature as well).

[Albab-Hasan]

@ChayimFriedman2 youre right. my mistake. shouldve done more research before making the pr. ill close it