add missing deduction guide for inplace_stop_callback

[kwy404]

The standard declares a deduction guide for inplace_stop_callback (stopcallback.inplace.general):

template<class CallbackFn>
  inplace_stop_callback(inplace_stop_token, CallbackFn) -> inplace_stop_callback<CallbackFn>;

stdexec did not declare it, so inplace_stop_callback cb{token, fn}; failed to compile. The constructor is a template over its own initializer type, so the implicit guide cannot deduce the callback type (MSVC: "error C2641: cannot deduce template arguments for 'stdexec::inplace_stop_callback'").

This adds the guide right after the class, plus a test in test_concepts_stop_tokens.cpp that checks the deduced type.

Fixes #1420

Tested locally with MSVC 14.44 (Debug): the new test fails to compile without the fix and passes with it, and the rest of test.stdexec passes too (test_any.cpp was left out of the local build because this MSVC version hits an internal compiler error on it, on main as well). clang-format reports no changes on the touched files.

[copy-pr-bot]

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