Validate CallbackTrigger callback path before running the callback

[potiuk]

Follow-up to #69792, which made the Triggerer check that a deferring task's trigger classpath resolves to a BaseTrigger subclass before using it.

CallbackTrigger resolves its callback_path with import_string and calls the result without checking what it is. This PR checks the callback path the same way trigger classpaths are checked. Before the callback runs, the resolved object must be one of:

• a coroutine function (async def),
• an awaitable callable, such as an async BaseNotifier subclass (has __await__),
• a callable object whose __call__ is async def.

The first two are what AsyncCallback.verify_callable already requires when a deadline callback is defined. Triggerer callbacks are always awaited, so every documented use is still accepted. Modules, non-callable attributes, plain classes and sync functions now fail the callback with a clear InvalidCallbackPathError and are not called. Before this change, a sync function was called and only then failed with a "not awaitable" error. Like any other callback failure, the error is reported as a FAILED callback event and the Triggerer keeps running.

Tests added:

• unit tests for _validate_callback, one case each for accepted and rejected targets,
• a CallbackTrigger.run test that resolves a real coroutine-function path,
• a CallbackTrigger.run test that checks invalid paths (module, non-callable attribute, sync callable, plain class) fail without the target being called.

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