[exec.stopped.opt] Add transform_sender tag

[abhinavagarwal07]

Pass set_value to stopped_as_optional.transform_sender in the not-a-sender branch, matching the tagged call used by the guard in the same paragraph.

[eisenwave]

Problem introduced with https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3557r3.html / #8035

The added wording says stopped_as_optional.transform_sender(sndr, env), but transform_sender has three parameters.

I think this came to be because P3557R3 cited an old version of pre-existing wording where it's still transform_sender(sndr, env), so LWG didn't notice that the new (green) wording should not have two arguments in the call expression, but three.

[eisenwave]

@abhinavagarwal07 in the future, could you provide the necessary historical context on these PRs?

It's important to know whether something is a misapplication of a paper or a problem introduced by paper/LWG issue, etc. The fixes are greatly appreciated though.