Add range-based when_all overloads and consolidate internals (#205)

Add when_all(Range&&) overloads for homogeneous non-void and void
awaitable ranges, matching the existing when_any range API. Extract
when_all_core and unify runner types to reduce internal duplication.

- Add IoAwaitableRange concept to concept/io_awaitable.hpp (shared
  with when_any)
- Add when_all_homogeneous_state<T>, when_all_homogeneous_state<void>,
  and when_all_homogeneous_launcher<Range> for range support
- Extract when_all_core from duplicated state members
- Unify when_all_runner and when_all_homogeneous_runner into single
  when_all_runner<StateType>
- Remove duplicate IoAwaitableRange definition from when_any.hpp
- Add when_all_range_test suite (11 tests)

Author: mvandeberg

include/boost/capy/concept/io_awaitable.hpp

@@ -13,6 +13,7 @@
 #include <boost/capy/detail/config.hpp>
 #include <coroutine>
 #include <boost/capy/ex/io_env.hpp>
+#include <ranges>
 
 namespace boost {
 namespace capy {
@@ -121,6 +122,19 @@ concept IoAwaitable =
 template<typename A>
 using awaitable_result_t = decltype(std::declval<std::decay_t<A>&>().await_resume());
 
+/** Concept for ranges of I/O awaitables.
+
+    A range satisfies `IoAwaitableRange` if it is a sized input range
+    whose value type satisfies @ref IoAwaitable.
+
+    @tparam R The range type.
+*/
+template<typename R>
+concept IoAwaitableRange =
+    std::ranges::input_range<R> &&
+    std::ranges::sized_range<R> &&
+    IoAwaitable<std::ranges::range_value_t<R>>;
+
 } // namespace capy
 } // namespace boost