Fix __receiver_proxy_base start_scheduler_t query.

[yulin25]

Current behaviour

When start scheduler is queried, __receiver_proxy_base does:

1. If the requested scheduler type is not task_scheduler, we just declare
   that there is no start scheduler, and the query returns nullopt.
2. Otherwise, the user is explicitly querying a start scheduler as task_scheduler.
   1. If there do exists a start scheduler, we give you the task_scheduler constructed
      from the start scheduler. And if the constructor throws, std::terminate() is automatically invoked as querying is noexcept.
   2. Otherwise, there is no start scheduler, but we still give you a task_scheduler created with inline_scheduler. Again, std::terminate() on exception.

There is one problem I encountered currently

This code does not compile:

namespace ex = STDEXEC;
auto sched = ex::get_parallel_scheduler();
ex::sync_wait(ex::starts_on(sched,ex::starts_on(sched,ex::just())));

That is parallel_scheduler being the start scheduler
which does not satisfies task_scheduler constructor's constraints, caused compilation failure.

task_scheduler is not named any_scheduler, which means some scheduler types can not be used to construct a task_scheduler, but 2.1 does not check that.

Fixes in this PR

1. If the start scheduler can not be used to construct a task_scheduler, and the user is explicitly requesting
   a task_scheduler, then there is no such start scheduler of the requested type.

2. According to the specification of receiver_proxy::try_query:

   Returns: Let env be the environment of the receiver represented by *this. If
   - Query is not a member of an implementation-defined set of supported queries; or
   - P is not a member of an implementation-defined set of supported result types for Query; or
   - the expression q(env) is not well-formed,
   then returns nullopt. Otherwise, if q(env) has type cv P, then returns q(env). Otherwise, returns an implementation-defined value of type optional<P>.

   1. Current implementation returns inline_scheduler when q(env)is not well-formed, which should be nullopt according to the standard.

   2. When the start scheduler's type is not task_scheduler, return an implementation-defined value of type optional<task_scheduler>:

      • If constructible as task_scheduler, return constructed task_scheduler.
      • Otherwise, return nullopt.

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