[promises] poll_cast improvements (#35302)

Make it possible to `poll_cast<>` from a normal non-poll type to a Poll<T>

Closes #35302

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35302 from ctiller:poll-review 216916e7c3
PiperOrigin-RevId: 590979951

src/core/lib/promise/poll.h

@@ -186,6 +186,21 @@ class Poll<Pending>;
 template <class T>
 class Poll<Poll<T>>;
 
+// PollTraits tells us whether a type is Poll<> or some other type, and is
+// leveraged in the PromiseLike/PromiseFactory machinery to select the
+// appropriate implementation of those concepts based upon the return type of a
+// lambda, for example (via enable_if).
+template <typename T>
+struct PollTraits {
+  static constexpr bool is_poll() { return false; }
+};
+
+template <typename T>
+struct PollTraits<Poll<T>> {
+  using Type = T;
+  static constexpr bool is_poll() { return true; }
+};
+
 template <typename T>
 bool operator==(const Poll<T>& a, const Poll<T>& b) {
   if (a.pending() && b.pending()) return true;
@@ -193,27 +208,37 @@ bool operator==(const Poll<T>& a, const Poll<T>& b) {
   return false;
 }
 
+template <typename T, typename U, typename SfinaeVoid = void>
+struct PollCastImpl;
+
 template <typename T, typename U>
-Poll<T> poll_cast(Poll<U> poll) {
+struct PollCastImpl<T, Poll<U>> {
+  static Poll<T> Cast(Poll<U>&& poll) {
     if (poll.pending()) return Pending{};
     return static_cast<T>(std::move(poll.value()));
-}
+  }
+};
+
+template <typename T, typename U>
+struct PollCastImpl<T, U, std::enable_if<!PollTraits<U>::is_poll()>> {
+  static Poll<T> Cast(U&& poll) { return Poll<T>(T(std::move(poll))); }
+};
 
-// PollTraits tells us whether a type is Poll<> or some other type, and is
-// leveraged in the PromiseLike/PromiseFactory machinery to select the
-// appropriate implementation of those concepts based upon the return type of a
-// lambda, for example (via enable_if).
 template <typename T>
-struct PollTraits {
-  static constexpr bool is_poll() { return false; }
+struct PollCastImpl<T, T> {
+  static Poll<T> Cast(T&& poll) { return Poll<T>(std::move(poll)); }
 };
 
 template <typename T>
-struct PollTraits<Poll<T>> {
-  using Type = T;
-  static constexpr bool is_poll() { return true; }
+struct PollCastImpl<T, Poll<T>> {
+  static Poll<T> Cast(Poll<T>&& poll) { return std::move(poll); }
 };
 
+template <typename T, typename U>
+Poll<T> poll_cast(U poll) {
+  return PollCastImpl<T, U>::Cast(std::move(poll));
+}
+
 // Convert a poll to a string
 template <typename T, typename F>
 std::string PollToString(