Detect async callable objects as awaitable handlers

[Sanjays2402]

Description

CallableObject.__post_init__ decided awaitability with inspect.iscoroutinefunction(callback), which is False for an instance of a class whose __call__ is a coroutine function. Such handlers/filters were therefore dispatched through asyncio.to_thread(), so calling them just returned an un-awaited coroutine ("coroutine was never awaited") and the handler body never ran. Awaitability now also considers type(callback).__call__, matching how Filter subclasses were already special-cased.

Note: the issue also mentions lambdas that return a coroutine. That is a plain sync function whose return value happens to be awaitable, which cannot be detected by introspection, so this PR intentionally only covers async callable objects.

Fixes #1721

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• New test_call_async_callable_object and the extended test_init_awaitable parametrization fail on dev-3.x (never-awaited coroutine returned) and pass with the fix.
• pytest tests/test_dispatcher — 133 passed, 1 skipped.

Test Configuration:

• Operating System: macOS
• Python version: 3.14

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[github-actions]

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